ldf.xsd

<xsd:schema
     xmlns:xsd="http://www.w3.org/2001/XMLSchema"
     xmlns:ldf="http://planet-sl.org/ldf"
     xmlns:bgf="http://planet-sl.org/bgf"
     xmlns:xhtml="http://www.w3.org/1999/xhtml"
     targetNamespace="http://planet-sl.org/ldf">

  <xsd:import namespace="http://planet-sl.org/bgf" schemaLocation="bgf.xsd"/>
  <xsd:import namespace="http://www.w3.org/1998/Math/MathML" schemaLocation="fakemathml.xsd"/>

  <!-- info -->
  <xsd:annotation>
    <xsd:documentation>
      Language Document Format
    </xsd:documentation>
    <xsd:documentation>
      This is basically a metamodel for language documents or documentations.

      Sample program tags:
      - External reference (with XPath-like subprogram selection)
      - Verbatim text + claim about corresponding grammar and nontermninal (and perhaps version)
      - Generation request for given grammar and nontermninal (and perhaps version, more control)
      - Sample suite extraction request (what are the constraints on the requested sample?)
    </xsd:documentation>
  </xsd:annotation>

  <!-- elements -->

  <xsd:element name="document">
    <xsd:annotation>
      <xsd:documentation>
        A document is essentially a sequence of several parts, such as title page, front matter,
        lexical and syntax sections, placeholders for generated content and various lists.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element name="titlePage" type="ldf:titlePage"/>
        <xsd:element name="frontMatter">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:group ref="ldf:topSection" minOccurs="0" maxOccurs="unbounded"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
        <xsd:element name="lists" minOccurs="0">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:group ref="ldf:frontList" maxOccurs="unbounded"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
        <xsd:element name="lexicalPart" minOccurs="0">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:group ref="ldf:lexicalSection" maxOccurs="unbounded"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
        <xsd:choice>
          <xsd:element name="core" type="ldf:structuredSection" maxOccurs="unbounded"/>
          <xsd:element name="part" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="core" type="ldf:structuredSection" maxOccurs="unbounded"/>
              </xsd:sequence>
            </xsd:complexType>
          </xsd:element>
        </xsd:choice>
        <xsd:element name="backMatter" minOccurs="0">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:group ref="ldf:topSection" minOccurs="0" maxOccurs="unbounded"/>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
        <xsd:element name="annex" type="ldf:structuredSection" minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>

  <xsd:complexType name="titlePage">
    <xsd:annotation>
      <xsd:documentation>
        The ``title page'' can in reality be rendered as several pages, but it contains the basic
        information that helps to identify this particular language definition and to distinguish
        it from similar documents. From our experience, language standards are either marked as
        organisation-created or person-authored ones. In the former case, the document must contain
        the name of the organisation and the standard reference number within it (e.g, ISO 10279).
        In the latter case, one or more authors are specified. It is also possible to mark some sections
        as having been authored by a specific set of authors, but the ones defined here are the principal
        authors that identify the specific standard.
      </xsd:documentation>
      <xsd:documentation>
        The ``topic'' of the language document is its pure textual title without the reference number
        and status: e.g., ``Programming Language REXX'',
        ``Information technology --- Programming languages --- Full BASIC'', etc. Either version or edition
        follows.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:choice>
        <xsd:sequence>
          <xsd:element name="body" type="ldf:body"/>
          <xsd:element name="number" type="xsd:int"/>
        </xsd:sequence>
        <xsd:sequence>
          <xsd:element name="author" type="xsd:string" maxOccurs="unbounded"/>
        </xsd:sequence>
      </xsd:choice>
      <xsd:element name="topic" type="xsd:string"/>
      <xsd:choice>
        <xsd:element name="version" type="xsd:string" default="1.0"/>
        <xsd:element name="edition" type="xsd:string"/>
      </xsd:choice>
      <xsd:element name="status" type="ldf:status"/>
      <xsd:element name="date" type="xsd:string"/>
      <!-- xsd:date -->
    </xsd:sequence>
  </xsd:complexType>

  <xsd:simpleType name="body">
    <xsd:restriction base="xsd:token">
      <xsd:enumeration value="ansi"/>
      <xsd:enumeration value="ecma"/>
      <xsd:enumeration value="ieee"/>
      <xsd:enumeration value="iec"/>
      <xsd:enumeration value="iso"/>
      <xsd:enumeration value="iso/iet"/>
      <xsd:enumeration value="itu"/>
      <xsd:enumeration value="ietf"/>
      <xsd:enumeration value="iec"/>
      <xsd:enumeration value="oasis"/>
      <xsd:enumeration value="wsa"/>
      <xsd:enumeration value="w3c"/>
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:simpleType name="status">
    <xsd:annotation>
      <xsd:documentation>
        According to W3C Process Document, each viable specification goes through the stages
        of Working Draft (WD), Candidate Recommendation (CR), Proposed Recommendation (PR),
        W3C Recommendation (REC), with possible continuation to Proposed Edited Recommendation
        and decline to Rescinded Recommendation. There has also been a ``Note'fma' status in the past
        for internal drafts.
      </xsd:documentation>
      <xsd:documentation>
        IEEE uses different publication types, they are:
        Changed Designation,
        Collection,
        New Standard Project,
        Modified New Project,
        Modified Revision Project,
        Revision Project,
        Adoption in Progress,
        Approved Publication of IEEE,
        International Publication,
        Trial Use,
        Amendment,
        Corrigenda of Standard,
        Superseded,
        Withdrawn (IEEE Status Report).
      </xsd:documentation>
      <xsd:documentation>
        ISO/IEC operates with the following standard ratification stages:
        Approved Work Item (AWI),
        Working Draft (WD), Committee Draft (CD),
        Committee Draft Technical Report (CD TR),
        Committee Draft Technical Specification (CD TS),
        Committee Draft for Vote (CDV, only for IEC),
        Draft International Standard (DIS),
        Final Committee Draft (FCD, only for JTC1),
        Draft Technical Report (DTR),
        Draft Technical Specification (DTS),
        Final Draft International Standard (FDIS),
        International Standard (ISO),
        Technical Report (ISO TR),
        Technical Specification (ISO TS).
      </xsd:documentation>
      <xsd:documentation>
        Some standardisation bodies like ANSI or ECMA do not have a long list of stages, the
        standard there is just either approved or not. Of course, it still can be revised,
        reaffirmed, withdrawn or be put into a category of technical reports.
      </xsd:documentation>
      <xsd:documentation>
        We summarise these and possibly other sets of statuses by the following enumeration.
        Conceptually it provides functionality for the same categories, but the concrete
        wording may vary (i.e., ``errata'' vs ``corrigenda'', ``obsolete'' vs ``rescinded'').
        If necessary, the schema can be extended with more standard publication types
        or even adapted to fit completely in some specific standardisation body classification.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:restriction base="xsd:token">
      <xsd:enumeration value="unknown"/>
      <xsd:enumeration value="draft"/>
      <!-- working draft, committee draft -->
      <xsd:enumeration value="candidate"/>
      <xsd:enumeration value="proposed"/>
      <xsd:enumeration value="approved"/>
      <!-- recommendation, international standard -->
      <xsd:enumeration value="revised"/>
      <xsd:enumeration value="obsolete"/>
      <xsd:enumeration value="withdrawn"/>
      <xsd:enumeration value="collection"/>
      <xsd:enumeration value="trial"/>
      <xsd:enumeration value="errata"/>
      <xsd:enumeration value="report"/>
    </xsd:restriction>
  </xsd:simpleType>


  <xsd:simpleType name="generated">
    <xsd:restriction base="xsd:token">
      <xsd:enumeration value="index"/>
      <xsd:enumeration value="fullgrammar"/>
      <xsd:enumeration value="listoftables"/>
      <xsd:enumeration value="listofauthors"/>
      <xsd:enumeration value="listofcontents"/>
      <xsd:enumeration value="listofreferences"/>
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:simpleType name="figureType">
    <xsd:restriction base="xsd:token">
      <xsd:enumeration value="PDF"/>
      <xsd:enumeration value="PostScript"/>
      <xsd:enumeration value="SVG"/>
      <xsd:enumeration value="PNG"/>
      <xsd:enumeration value="GIF"/>
      <xsd:enumeration value="JPEG"/>
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:group name="topSection">
    <xsd:choice>
      <xsd:element name="foreword" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Whatever the authors deem to be important enough to be put on one of the first
            pages. For example, in C\# specifications Foreword is about the differences brought
            to the language by the current standard, while in the Scheme specification
            Foreword discusses programming languages design and demonstrates its principles
            applied to the forthcoming document.
          </xsd:documentation>
          <xsd:documentation>
            Technically speaking, Foreword is not a part of the specification. Instead, it
            precedes the specification and introduces it by putting in the proper context.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="designGoals" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            The goals of language design are sometimes encounted being explicitly
            stated in the language document in one of the informative sections
            of the front matter part.
          </xsd:documentation>
          <xsd:documentation>
            For example:
            "C\# is intended to be a simple, modern, general-purpose, object-oriented
            programming language."
            (from ECMA 334 3rd edition, page xvii)
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="scope" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Scope section explains the context for the language document.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="conformance" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Conformance section defines several levels of compliance by
            explaining what is a conforming program and a conforming
            implementation with respect to this standard.
          </xsd:documentation>
          <xsd:documentation>
            Definitions for meta-terms like "shall" and "should"
            and their relation to the compliance issue explained above.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="compliance" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Same as conformance
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="compatibility" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            While conformance/compliance define how external artifacts should
            conform to this standard, this section defines how this standard
            complies with previously existing ones.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="notation" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Notation section defines grammar definition formalism used in the document:
            mostly it is about the EBNF dialect.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="normativeReferences" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Formally lined up references to all other standards that are used or referenced to
            from within the document.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="documentStructure" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            This section informally describes how the document is organised, divided
            into parts and chapters. Sometimes it explicitly states which sections are
            normative and which are informative.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="whatsnew" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            A list of changes brought to the language by the current specification
            replacing the previous one.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="placeholder" type="ldf:generated">
        <xsd:annotation>
          <xsd:documentation>
            Placeholders are used to designate places where generated content
            should be inserted.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <!-- none of the above 
      <xsd:element name="core" type="ldf:simpleSection"/>
      -->
    </xsd:choice>
  </xsd:group>


  <xsd:complexType name="simpleSection">
    <xsd:sequence>
      <xsd:element name="title" type="xsd:string" minOccurs="0"/>
      <xsd:sequence>
        <xsd:element name="author" type="xsd:string" minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
      <xsd:element name="content" type="ldf:simpleText"/>
    </xsd:sequence>
    <xsd:attribute name="id" type="xsd:ID" use="optional"/>
  </xsd:complexType>

  <xsd:complexType name="simpleText">
    <xsd:sequence>
      <xsd:group ref="ldf:simpleTextElement" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:group name="simpleTextElement">
    <xsd:choice>
      <xsd:element name="empty" nillable="true">
        <xsd:complexType/>
      </xsd:element>
      <xsd:element name="text" type="ldf:mixedType"/>
      <xsd:element name="figure" type="ldf:simpleFigure"/>
      <xsd:element name="table" type="ldf:simpleTable"/>
      <xsd:element name="list" type="ldf:simpleList"/>
      <xsd:element name="formula">
        <xsd:complexType>
          <xsd:sequence maxOccurs="unbounded">
            <xsd:any namespace="http://www.w3.org/1998/Math/MathML"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
      <!--<xsd:element name="sample">
        <xsd:complexType>
          <xsd:simpleContent>
            <xsd:extension base="xsd:string">
              <xsd:attribute name="src" type="xsd:string"/>
            </xsd:extension>
          </xsd:simpleContent>
        </xsd:complexType>
      </xsd:element>-->
      <xsd:element name="sample">
        <xsd:complexType mixed="true">
          <xsd:sequence minOccurs="0" maxOccurs="unbounded">
            <xsd:any namespace="http://www.w3.org/1999/xhtml" processContents="lax"/>
          </xsd:sequence>
          <xsd:attribute name="src" type="xsd:string"/>
        </xsd:complexType>
      </xsd:element>
      <xsd:element ref="bgf:production"/>
      <!-- 	later redo with MathML -->
      <!--<xsd:element ref="mml:math" />-->
    </xsd:choice>
  </xsd:group>

  <xsd:complexType name="simpleFigure">
    <xsd:sequence>
      <xsd:element name="shortcaption" type="xsd:string" minOccurs="0"/>
      <xsd:element name="caption" type="xsd:string"/>
      <xsd:element name="source" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="type" type="ldf:figureType"/>
            <xsd:choice>
              <xsd:element name="localfile" type="xsd:string"/>
              <xsd:element name="url" type="xsd:anyURI"/>
            </xsd:choice>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
    <xsd:attribute name="id" type="xsd:ID" use="optional"/>
  </xsd:complexType>

  <xsd:complexType name="simpleTable">
    <xsd:sequence>
      <xsd:element name="header" type="ldf:tableRow" minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="row" type="ldf:tableRow" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="simpleList">
    <xsd:sequence>
      <xsd:element name="item" type="ldf:mixedType" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:group name="frontList">
    <xsd:choice>
      <xsd:element name="definitions" type="ldf:listOfTerms">
        <xsd:annotation>
          <xsd:documentation>
            This is a list of definitions for all basic terms needed to understand the document,
            but not especially introduced in it. Typical examples include explaining what is
            a "program", what is a "namespace", what is a "library", what is "behaviour".
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="abbreviations" type="ldf:listOfTerms">
        <xsd:annotation>
          <xsd:documentation>
            Acronyms like IEEE, ISO or like CLI, BCL are frequently defined here in a separate definition list.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="languageOverview" type="ldf:listOfTerms">
        <xsd:annotation>
          <xsd:documentation>
            Such an informal introduction to the language is not present in all standards.
            However, some specifications contain a not claimed to be complete list of
            language concepts with their definitions and perhaps even some examples.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:choice>
  </xsd:group>

  <xsd:group name="lexicalSection">
    <xsd:annotation>
      <xsd:documentation>
        Sections describing lexical structure tend to be shorter, less structured inside,
        and very limited in scope: there is usually one on whitespace, one on tokens,
        one on literals, one on comments, etc.
      </xsd:documentation>
      <xsd:documentation>
        See the section on grammar notation for more details about how broad even the smallest
        aspects (e.g., about line continuations) can vary.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:choice>
      <xsd:element name="lineContinuations" type="ldf:simpleSection"/>
      <xsd:element name="whitespace" type="ldf:simpleSection"/>
      <xsd:element name="tokens" type="ldf:simpleSection"/>
      <xsd:element name="preprocessor" type="ldf:simpleSection"/>
      <xsd:element name="literals" type="ldf:simpleSection"/>
      <!-- none of the above -->
      <xsd:element name="lexical" type="ldf:simpleSection"/>
    </xsd:choice>
  </xsd:group>

  <xsd:complexType name="structuredSection">
    <xsd:annotation>
      <xsd:documentation>
        The main bulk of the information the language document possesses
        lies in its core sections: they have more structure than other sections
        and than their own subsections, so we call them ``structured sections''.
      </xsd:documentation>
      <xsd:documentation>
        Beside the title, any section can include a list of authors that worked on it:
        this way even if the whole document is attributed to a particular company
        or a standardisation body, the main contributors can be attributed.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="title" type="xsd:string"/>
      <xsd:sequence>
        <xsd:element name="author" type="xsd:string" minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
      <xsd:group ref="ldf:structuredSectionElement" maxOccurs="unbounded" />
    </xsd:sequence>
    <xsd:attribute name="id" type="xsd:ID" use="optional"/>
  </xsd:complexType>



  <!-- types of content -->
  <xsd:complexType name="listOfTerms">
    <xsd:sequence>
      <xsd:element name="title" type="xsd:string" minOccurs="0"/>
      <xsd:sequence>
        <xsd:element name="author" type="xsd:string" minOccurs="0" maxOccurs="unbounded"/>
      </xsd:sequence>
      <xsd:element name="term" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="name" type="xsd:string"/>
            <xsd:element name="definition" type="ldf:simpleText"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
    <xsd:attribute name="id" type="xsd:ID" use="optional"/>
  </xsd:complexType>

  <!--<xsd:complexType name="listOfInternalLinks">
    <xsd:sequence>
      <xsd:element name="link" maxOccurs="unbounded">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="text" type="xsd:string"/>
            <xsd:element name="linkText" type="xsd:string" minOccurs="0"/>
            <xsd:element name="reference" type="xsd:IDREF"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>-->

  <xsd:complexType name="tableRow">
    <xsd:sequence>
      <xsd:element name="cell" maxOccurs="unbounded" type="ldf:simpleText"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:group name="structuredSectionElement">
    <xsd:choice>
      <xsd:element name="placeholder" type="ldf:generated">
        <xsd:annotation>
          <xsd:documentation>
            Some subsections can be generated, especially those that consist
            of structured content that is possible to derive automatically
            from the information stored in or collected from other sections.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:group ref="ldf:normative">
        <xsd:annotation>
          <xsd:documentation>
            Normative sections are obligatory and usually contain strict content
            that needs to be implemented by compiler developers or satisfied
            by language end users in order to comply to this standard.
          </xsd:documentation>
          <xsd:documentation>
            For instance, if a section with a grammar production is marked as
            normative, this production must hold for the implemented language.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:group>
      <xsd:group ref="ldf:informative">
        <xsd:annotation>
          <xsd:documentation>
            Informative sections are supplementary and provide some useful information
            that can be omitted or overridden if deemed appropriately.
          </xsd:documentation>
          <xsd:documentation>
            For instance, if all sections with code samples are marked as informative,
            we cannot rely on the set of examples extracted from the standard to be
            the test set for the language. Apparently, we can still run some analyses
            on the basis of this, but it is not legitimate to make any conclusions
            about standard inconsistencies based on the informative sections, nor can
            they be legitimately be used to resolve inner conflicts of the documentation.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:group>
      <xsd:element ref="bgf:production">
        <xsd:annotation>
          <xsd:documentation>
            Grammar productions can be easily extracted from the language document
            for the purpose of grammar recovery, grammar adaptation, grammar convergence, etc.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="references" type="ldf:simpleList">
        <xsd:annotation>
          <xsd:documentation>
            Hardbound standards have been trying to be more hypertext
            for decades. One of the ways was to attach a list of links
            and backlinks to every section --- that way, one can easily
            find any related language constructs when learning the
            language or debugging a particular feature.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="section" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Any other type of normative or informative section.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <!-- <xsd:group ref="ldf:top-section"/> -->
      <xsd:element name="subtopic" type="ldf:structuredSection">
        <xsd:annotation>
          <xsd:documentation>
            In big documents it is not uncommon to find one topic divided into
            several subtopics, each one dedicated to a separate issue and each one
            structured in the same way its parent section is.
            DSLs and 4GLs specification authors often find it easier to lay out
            different clauses of one language construct in different sections.
            When the language has a lof of parametrised constructs, it makes sense
            to dedicate a special subsection for each field.
            Parameters, types, methods, operations, participants---whatever categories
            inspire these subtopics, each of them is a fully structured section in itself.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="value">
        <xsd:annotation>
          <xsd:documentation>
            A singled named value can be bound to a language document section: it can be
            a type of a parameter that is being described here, or an alternative name,
            or a superclass, or anything else that is atomic and non-structured.
          </xsd:documentation>
        </xsd:annotation>
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="key" type="xsd:string"/>
            <xsd:element name="data" type="xsd:string"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:choice>
  </xsd:group>

  <!-- tokens -->

  <xsd:group name="normative">
    <xsd:annotation>
      <xsd:documentation>
        Normative sections form the core of the language standard: for each core section they belong to,
        they define the purpose of the language construct, provide a description, some syntax definition,
        list use constraints and other semantic details, etc.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:choice>
      <xsd:element name="synopsis" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            A separate subsection named ``purpose'' is only encountered in some 4GL
            language manuals (e.g., JCL). However, it is quite common for the first
            paragraph of any new section of any language document to relay short word
            about the purpose of the language construct that is about to be introduced.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="description" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Description is the core of the parent section, containing the main details about the
            defined topic, information about its usage, motivation behind its design.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="syntax" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Syntax sections consist of one or more BGF productions, possibly complemented by textual descriptions.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="constraints" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            This section can list requirements needed for using a specific language construct,
            applicability constraints and other restrictions.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="relationship" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Related language constructs can be named and referenced here.
            It is not a simple list of references, but rather a comprehensive
            overview on the kind of consequences other parts of the language
            can bear if this one is used.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="semantics" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            A section on semantics explains in plain English, if no other specific formalism
            is used, how exactly the language construct works, what happens inside the system when
            it is utilised. It also describes the context in which the introduced language construct
            can be encountered and in which it should or should not be used.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="default" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            It is quite common in the sections that describe an optional parameter
            to tell the reader what will happen in the case nothing was specified.
            ``There is no default'' can be as valid a definition as a real default value.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <!-- none of the above 
      <xsd:element name="normative" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            Any other type of normative section.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>-->
    </xsd:choice>
  </xsd:group>

  <xsd:group name="informative">
    <xsd:annotation>
      <xsd:documentation>
        Unlike normative sections that impose some conformance constraints that need
        to be satisfied by a language processor to claim compatibility with the standard,
        the informative sections are only presented to provide some additional information
        to the reader.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:choice>
      <xsd:element name="rationale" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            A rationale or a note usually lists some narrow places of
            non-obvious usage, implementation details, incompatibility issues,
            coding standards, common programming practices, etc. It is a
            subsection of secondary importance, contributing some information
            on a minor point that can still be interesting and useful for some
            readers. Notes usually tell the readers how to use certain language
            constructs or tell compiler vendors how to implement them.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="example" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            A subsection with an example can contain a code sample as well
            as some accompanying text.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="update" type="ldf:simpleSection">
        <xsd:annotation>
          <xsd:documentation>
            It is quite uncommon practice, but some standardisation bodies really
            put the information about language evolution directly into each section
            that changed since the last public version of the language document.
          </xsd:documentation>
          <xsd:documentation>
            If this is done consistently and carefully, it is possible to generate
            the global ``What's new'' section automatically.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <!-- none of the above -->

    </xsd:choice>
  </xsd:group>

  <!--xsd:complexType name="contentType">
    <xsd:annotation>
      <xsd:documentation>
        A sections content is a list of different kinds of portions.
        Text is one kind of portion.
        A grammar fragment is another kind of portion.
        A sample is yet another kind of portion.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:choice minOccurs="0" maxOccurs="unbounded">
        <xsd:element name="text" type="ldf:mixedType"/>
        <xsd:element name="grammar">
          <xsd:complexType>
            <xsd:sequence>
              <xsd:any namespace="http://planet-sl.org/bgf" maxOccurs="unbounded"/>
            </xsd:sequence>
            <xsd:attribute name="language" type="xsd:anyURI" use="required"/>
            <xsd:attribute name="version" type="xsd:token" use="optional"/>
          </xsd:complexType>
        </xsd:element>
        <xsd:element name="sample" type="ldf:informative"/>
        <xsd:element name="runnable" type="ldf:runnableType"/>
      </xsd:choice>
      <xsd:element name="section" type="ldf:section" minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType-->

  <xsd:complexType name="runnableType">
    <xsd:annotation>
      <xsd:documentation>
        Related to the sample, this element shows a way to run a sample.
        In FL, it is always just a name of one of the functions and the list of its parameters.
        Generally, while keeping the internal structure of this element as such,
        one might want to represent it in the resulting language document differently
        (i.e. "java Test a b c" instead of "main(a,b,c);" ).
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="context" type="xsd:IDREF">
        <xsd:annotation>
          <xsd:documentation>
            The reference to another sample that must be used
            as a context while evaluating this one
            The means to refer to this particular sample later in the same document.
            For example, this sample introduces a function that is used in one of the next samples.
            It can also be a full text of a program that is run with different set of attributes later.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="main" type="xsd:string">
        <xsd:annotation>
          <xsd:documentation>
            The main function that is applied to the arguments.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="argument" type="xsd:string" minOccurs="0" maxOccurs="unbounded">
        <xsd:annotation>
          <xsd:documentation>
            These arguments are passed to the function during execution.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
      <xsd:element name="yields" type="xsd:string">
        <xsd:annotation>
          <xsd:documentation>
            The value that the sample should yield if parsed and
            evalutated in the specified language.
          </xsd:documentation>
        </xsd:annotation>
      </xsd:element>
    </xsd:sequence>
    <xsd:attribute name="language" type="xsd:anyURI" use="required"/>
    <xsd:attribute name="version" type="xsd:token" use="optional"/>
    <xsd:attribute name="id" type="xsd:ID" use="optional">
      <xsd:annotation>
        <xsd:documentation>
          The means to refer to this particular sample later in the same document.
          For example, this sample introduces a function that is used in one of the next samples.
          It can also be a full text of a program that is run with different set of attributes later.
        </xsd:documentation>
      </xsd:annotation>
    </xsd:attribute>
  </xsd:complexType>

  <!--xsd:complexType name="sectionTypeodl">
    <xsd:annotation>
      <xsd:documentation>
        A section has a title, some content, and possibly a list of subsections.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence>
      <xsd:element name="title" type="xsd:string"/>
      <xsd:element name="content" type="ldf:contentType"/>
    </xsd:sequence>
  </xsd:complexType-->

  <xsd:complexType name="mixedType" mixed="true">
    <xsd:annotation>
      <xsd:documentation>
        Any unstructured element of the language document belongs to a so called
        mixed type: i.e., it is plain text with some keywords marked.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:sequence minOccurs="0" maxOccurs="unbounded">
      <xsd:any namespace="http://planet-sl.org/ldf" processContents="lax"/>
    </xsd:sequence>
    <!--<xsd:anyAttribute namespace="##other" processContents="lax"/>-->
  </xsd:complexType>

  <xsd:element name="formula">
    <xsd:annotation>
      <xsd:documentation>
        Formulae can be used in language documentation in many ways.
        The internal representation format is MathML which is W3C Recommendation,
        but the external representation can vary, in our case there are two:
        \TeX\ and HTML.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexType>
      <xsd:sequence maxOccurs="unbounded">
        <xsd:any namespace="http://www.w3.org/1998/Math/MathML"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>

  <xsd:element name="keyword" type="xsd:string">
    <xsd:annotation>
      <xsd:documentation>
        Keywords are usually printed in bolder font weight.
        They need to be marked as such for two purposes: for presentation and for meta-information.
        The former goal serves as a basis for typesetting and hyperlinking,
        while the latter allows for correct indexing and searching facilities.
      </xsd:documentation>
    </xsd:annotation>
  </xsd:element>

  <xsd:element name="code" type="xsd:string">
    <xsd:annotation>
      <xsd:documentation>
        Inlined pieces of code are usually printed in typewriter-like font.
        They are frequently incomplete, mostly nothing more than simple literals,
        and can only be checked to be correct tokens of the language in
        a lexical sense.
      </xsd:documentation>
    </xsd:annotation>
  </xsd:element>

  <xsd:element name="link">
    <xsd:annotation>
      <xsd:documentation>
        Internal links are pairs of text that will become clickable in hypertext
        presentation forms or precede the reference itself when this is the only
        option. The reference points to a section or a subsection of the same
        document that the link should refer to. If the explicit text is omitted,
        its default value is the name of the section being referenced.
      </xsd:documentation>
    </xsd:annotation>
    <xsd:complexType>
      <xsd:sequence>
        <!--<xsd:element name="text" type="xsd:string"/>-->
        <xsd:element name="text" type="xsd:string" minOccurs="0"/>
        <xsd:element name="reference" type="xsd:IDREF"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>

</xsd:schema>