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appendix-IV: Refactor the license expression appendix
This is a large diff, but I aimed for restructuring/polishing without changing the end result. I did make a few minor, intentional changes: * Extended license-id to include appendix I.3 (deprecated licenses). We don't want folks using these in license expressions (because they're deprecated), but they are valid (or we would have removed them instead of just deprecating them). That means that in some cases the nature of a string is unclear. For example 'GPL-1.0+' could be the depreacted license-id, or it could be a simple-expression using the non-deprecated GPL-1.0 license-id and the + operator. I don't think that's a problem though, because I can't think of a case where the ambiguity would matter. * I've allowed + for license-ref (it used to be only for license-id). There could be external licenses which offer a choice between only-this-version and or-later grants, and allowing + for license-ref makes it easier to support those licenses as they transition into the SPDX License List. This isn't a big deal, but it avoids needing separate license-refs for the only-this-version and or-later grants if you need both. * I've added explicit whitespace handling, vs. the previous version which just discussed it in the text. That way the ABNF is the sole source of normative syntax information. * I've added enclosed-license-expression, so consumers like the tag:value format can suggest/require it. This allows for more precision in consumers (e.g. appendix V should be updated to require enclosed-license-expression), but I've left those other sections alone for this commit. Ideally the tag:value line would be moved to a separate section that defined the tag-value format, but we don't have such a section yet [1]. Also, it seems odd that there's no way to define an external license exception (LicenseExceptionRef?). But I haven't touched that in this commit. The RDF/XML bindings seem to have some holes around the + and WITH operators. I've left some FIXMEs where I think we need adjustments. The HTML comment avoids the ambiguous four-space indent after the list. Without the comment, it could be parsed as a code block (which is what we want) [2] or a second paragraph of the final list entry [3] (which is not what we want). The HTML comment closes the list to resolve the ambiguity. [1]: #22 (comment) [2]: https://daringfireball.net/projects/markdown/syntax#precode "To produce a code block in Markdown, simply indent every line of the block by at least 4 spaces or 1 tab" [3]: https://daringfireball.net/projects/markdown/syntax#list "Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab"
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| # SPDX License Expressions | ||
| # Appendix IV: SPDX License Expressions | ||
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| ## Overview | ||
| ## IV.1 Overview <a name="IV.1"></a> | ||
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| Often a single license can be used to represent the licensing terms of a source code or binary file, but there are situations where a single license identifier is not sufficient. A common example is when software is offered under a choice of one or more licenses (e.g., GPL-2.0 OR BSD-3-Clause). Another example is when a set of licenses is needed to represent a binary program constructed by compiling and linking two (or more) different source files each governed by different licenses (e.g., LGPL-2.1 AND BSD-3-Clause). | ||
| Often a single license can be used to represent the licensing terms of a source code or binary file, but there are situations where a single license identifier is not sufficient. A common example is when software is offered under a choice of one or more licenses (e.g., `GPL-2.0 OR BSD-3-Clause`). Another example is when a set of licenses is needed to represent a binary program constructed by compiling and linking two (or more) different source files each governed by different licenses (e.g., `LGPL-2.1 AND BSD-3-Clause`). | ||
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| SPDX License Expressions provides a way for one to construct expressions that more accurately represent the licensing terms typically found in open source software source code. A license expression could be a single license identifier found on the SPDX License List; a user defined license reference denoted by the LicenseRef-{idString}; a license identifier combined with an SPDX exception; or some combination of license identifiers, license references and exceptions constructed using a small set of defined operators (e.g., AND, OR, WITH and +). We provide the definition of what constitutes a valid an SPDX License Expression in this section. | ||
| SPDX License Expressions allow accurate representations of the licensing terms typically found in open source software. A license expression could be a single license identifier found on the [SPDX License List](appendix-I-SPDX-license-list.md); a user defined license reference denoted by the `LicenseRef-{idString}`; a license identifier combined with an SPDX exception; or some combination of license identifiers, license references and exceptions constructed using a small set of defined operators (`AND`, `OR`, `WITH` and `+`). We provide the definition of what constitutes a valid an SPDX License Expression in this section. | ||
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| The exact syntax of license expressions is described below in [ABNF](http://tools.ietf.org/html/rfc5234). | ||
| ## IV.2 Syntax <a name="IV.2"></a> | ||
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| idstring = 1*(ALPHA / DIGIT / "-" / "." ) | ||
| The exact syntax of license expressions is described below in [ABNF][rfc5234]. `ALPHA`, `DIGIT`, and `WSP` are from the [ABNF core rules][rfc5234-aB]. | ||
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| license-id = \<short form license identifier in [Appendix I.1](./appendix-I-SPDX-license-list.md)> | ||
| ``` | ||
| idstring = 1*(ALPHA / DIGIT / "-" / "." ) | ||
| license-id = <short form license identifier> | ||
| license-exception-id = <short form license exception identifier> | ||
| license-ref = ["DocumentRef-" idstring ":"] "LicenseRef-" idstring | ||
| license = license-id / license-ref | ||
| space = 1*WSP | ||
| simple-expression = license ["+"] [space "WITH" space license-exception-id] | ||
| binary-compound-operator = "AND" / "OR" | ||
| license-expression = simple-expression | ||
| / license-expression space binary-license-operator space license-expression | ||
| / "(" license-expression ")" | ||
| / space license-expression | ||
| / license-expression space | ||
| enclosed-license-expression = [space] simple-expression [space] | ||
| / [space] "(" [space] license-expression [space] ")" [space] | ||
| ``` | ||
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| license-exception-id = \<short form license exception identifier in [Appendix I.2](appendix-II-license-matching-guidelines-and-templates.md)> | ||
| Where: | ||
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| license-ref = ["DocumentRef-"1\*(idstring)":"]"LicenseRef-"1*(idstring) | ||
| * Short form license identifiers are drawn from sections [I.1](appendix-I-SPDX-license-list.md#I.1) and [I.3](appendix-I-SPDX-license-list.md#I.3). | ||
| * Short form license exception identifiers are drawn from [section I.2](appendix-I-SPDX-license-list.md#I.2). | ||
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| simple-expression = license-id / license-id”+” / license-ref | ||
| When a particular rule is not referenced, “SPDX License Expressions” means the more general `license-expression`. | ||
| Consumers that need the more limited `enclosed-license-expression` (which may be easier to parse depending on the surrounding context) SHOULD say so explicitly. | ||
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| compound-expression = 1*1(simple-expression / | ||
| Tag:value authors SHOULD use `enclosed-license-expression` instead of the more general `license-expression`. | ||
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| ### IV.2.1 The `+` Operator <a name="IV.2.1"></a> | ||
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| simple-expression "WITH" license-exception-id / | ||
| ### IV.2.2 Exception `WITH` Operator <a name="IV.2.2"></a> | ||
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| compound-expression "AND" compound-expression / | ||
| Sometimes a set of license terms apply except under special circumstances. In this case, use the binary `WITH` operator to construct a new simple expression to represent the exception situation. A valid `simple-expression` is where the left operand is a `license` value (with an optional `+` suffix) and the right operand is a `license-exception-id` that represents the exception terms. | ||
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| compound-expression "OR" compound-expression ) / | ||
| For example, when the Bison exception is applied to `GPL-2.0+`, the `simple-expression` would be: | ||
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| "(" compound-expression ")" ) | ||
| GPL-2.0+ WITH Bison-exception-2.2 | ||
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| license-expression = 1*1(simple-expression / compound-expression) | ||
| The current set of valid exceptions can be found in [Appendix I, section 2](appendix-I-SPDX-license-list.md#I.2). For the most up to date set of exceptions please see [spdx.org/licenses](https://spdx.org/licenses). If the applicable exception is not found on the SPDX License Exception List, then use a single `license-ref` to represent the entire license terms (including the exception). | ||
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| In the following sections we describe in more detail \<license-expression> construct, a licensing expression string that enables a more accurate representation of the licensing terms of modern day software. | ||
| ### IV.2.3 Conjunctive `AND` Operator <a name="IV.2.3"></a> | ||
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| A valid \<license-expression> string consists of either: | ||
| If required to simultaneously comply with two or more licenses, use the conjunctive binary `AND` operator to construct a new license expression, where both the left and right operands are valid `license-expression` values. | ||
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| (i) a simple license expression, such as a single license identifier; or | ||
| For example, when one is required to comply with both the `LGPL-2.1` or `MIT` licenses, a valid `license-expression` would be: | ||
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| (ii) a more complex expression constructed by combining smaller valid expressions using Boolean license operators. | ||
| LGPL-2.1 AND MIT | ||
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| There MUST NOT be whitespace between a license-id and any following "+". This supports easy parsing and backwards compatibility. There MUST be whitespace on either side of the operator "WITH". There MUST be whitespace and/or parentheses on either side of the operators "AND" and "OR". | ||
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| ## Simple License Expressions <a name="simple-expr"></a> | ||
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| A simple `<license-expression>` is composed one of the following: | ||
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| * An SPDX License List Short Form Identifier. For example: GPL-2.0 | ||
| * An SPDX License List Short Form Identifier with a unary"+" operator suffix to represent the current version of the license or any later version. For example: GPL-2.0+ | ||
| * A SPDX user defined license reference: ["DocumentRef-"1\*(idstring)":"]"LicenseRef-"1*(idstring) | ||
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| Some examples: | ||
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| LicenseRef-23 | ||
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| LicenseRef-MIT-Style-1 | ||
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| DocumentRef-spdx-tool-1.2:LicenseRef-MIT-Style-2 | ||
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| ## Composite License Expressions <a name="composite-expr"></a> | ||
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| More expressive composite license expressions can be constructed using "OR", "AND", and "WITH" operators similar to constructing mathematical expressions using arithmetic operators. For the Tag:value format, any license expression that consists of more than one license identifier and/or LicenseRef, should be encapsulated by parentheses: "( )". This has been specified to facilitate expression parsing. Nested parentheses can also be used to specify an order of precedence which is discussed in more detail in subsection (4). | ||
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| ### 1) Disjunctive "OR" Operator | ||
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| If presented with a choice between two or more licenses, use the disjunctive binary "OR" operator to construct a new lincense expression, where both the left and right operands are valid license expression values. | ||
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| For example, when given a choice between the LGPL-2.1 or MIT licenses, a valid expression would be: | ||
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| (LGPL-2.1 OR MIT) | ||
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| An example representing a choice between three different licenses would be: | ||
| An example where all three different licenses apply would be: | ||
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| (LGPL-2.1 OR MIT OR BSD-3-Clause) | ||
| LGPL-2.1 AND MIT AND BSD-2-Clause | ||
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| ### 2) Conjunctive "AND" Operator | ||
| ### IV.2.4 Disjunctive `OR` Operator <a name="IV.2.4"></a> | ||
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| If required to simultaneously comply with two or more licenses, use the conjunctive binary "AND" operator to construct a new license expression, where both the left and right operands are a valid license expression values. | ||
| If presented with a choice between two or more licenses, use the disjunctive binary `OR` license operator to construct a new license expression, where both the left and right operands are valid `license-expression` values. | ||
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| For example, when one is required to comply with both the LGPL-2.1 or MIT licenses, a valid expression would be: | ||
| For example, when given a choice between the `LGPL-2.1` or `MIT` licenses, a valid `license-expression` would be: | ||
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| (LGPL-2.1 AND MIT) | ||
| LGPL-2.1 OR MIT | ||
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| An example where all three different licenses apply would be: | ||
| An example representing a choice between three different licenses would be: | ||
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| (LGPL-2.1 AND MIT AND BSD-2-Clause) | ||
| LGPL-2.1 OR MIT OR BSD-3-Clause | ||
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| ### 3) Exception "WITH" Operator | ||
| ### IV.2.5 Grouping <a name="IV.2.5"></a> | ||
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| Sometimes a set of license terms apply except under special circumstances. In this case, use the binary "WITH" operator to construct a new license expression to represent the special exception situation. A valid \<license-expression> is where the left operand is a \<simple-expression> value and the right operand is a \<license-exception-id> that represents the special exception terms. | ||
| Parentheses (`()`) can be used to specify an explicit grouping to override [operator precedence](#IV.2.6). This is similar to the use of parentheses in algebraic expressions like `(5+7)/2`. | ||
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| For example, when the Bison exception is to be applied to GPL-2.0+, the expression would be: | ||
| For instance, the following expression: | ||
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| (GPL-2.0+ WITH Bison-exception-2.2) | ||
| MIT AND (LGPL-2.1+ OR BSD-3-Clause) | ||
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| The current set of valid exceptions can be found in [Appendix I, section 2](appendix-I-SPDX-license-list.md#I.2). For the most up to date set of exceptions please see [spdx.org/licenses](https://spdx.org/licenses). If the applicable exception is not found on the SPDX License Exception List, then use a single \<license-ref> to represent the entire license terms (including the exception). | ||
| represents a license requring both `MIT` and the expression `LGPL-2.1+ OR BSD-3-Clause`, because the parenthesis require the enclosed expression take precedence over (is applied before) the `AND` operator. | ||
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| ### 4) Order of Precedence and Parentheses | ||
| ### IV.2.6 Operator Precedence <a name="IV.2.6"></a> | ||
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| The order of application of the operators in an expression matters (similar to mathematical operators). The default operator order of precedence of a \<license-expression> a is: | ||
| The operators described above have the following precedence, from highest (binding tightest) at the top, to lowest (loosest) at the bottom: | ||
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| Grouping | ||
| + | ||
| WITH | ||
| AND | ||
| OR | ||
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| where a lower order operator is applied before a higher order operator. | ||
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| For example, the following expression: | ||
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| LGPL-2.1 OR BSD-3-Clause AND MIT | ||
| LGPL-2.1 OR BSD-3-Clause AND MIT | ||
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| represents a license choice between either LGPL-2.1 and the expression BSD-3-Clause AND MIT because the AND operator takes precedence over (is applied before) the OR operator. | ||
| represents a license choice between either `LGPL-2.1` and the expression `BSD-3-Clause AND MIT` because the `AND` operator takes precedence over (is applied before) the `OR` operator. | ||
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| When required to express an order of precedence that is different from the default order a \<license-expression> can be encapsulated in pairs of parentheses: ( ), to indicate that the operators found inside the parentheses takes precedence over operators outside. This is also similar to the use of parentheses in an algebraic expression e.g., (5+7)/2. | ||
| ## IV.3 License Expressions in XML <a name="IV.3"></a> | ||
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| For instance, the following expression: | ||
| ### IV.3.1 The `+` Operator <a name="IV.3.1"></a> | ||
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| FIXME: How are you supposed to cover this in XML? | ||
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| (MIT AND (LGPL-2.1+ OR BSD-3-Clause)) | ||
| ### IV.3.2 Exception `WITH` Operator <a name="IV.3.2"></a> | ||
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| states the OR operator should be applied before the AND operator. That is, one should first select between the LGPL-2.1+ or the BSD-3-Clause license before applying the MIT license. | ||
| A License Exception can be expressed in XML via a `<spdx:LicenseException>` element. This element has the following children: | ||
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| ### 5) License Expressions in RDF <a name="rdf-expr"></a> | ||
| * [`rdfs:comment`][rdfs:comment] element describing the nature of the exception. | ||
| * [`rdfs:seeAlso`][rdfs:seeAlso] element (optional) referencing external sources of information on the exception. | ||
| * `spdx:example`: Text describing examples of this exception. | ||
| * `spdx:name`: The full human readable name of the item. | ||
| * `spdx:licenseExceptionId`: The identifier of an exception in the SPDX License List to which the exception applies. | ||
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| A conjunctive license can be expressed in RDF via a `<spdx:ConjunctiveLicenseSet>` element, with an spdx:member property for each element in the conjunctive license. Two or more members are required. | ||
| FIXME: I don't think this element makes sense. What you want is a way to represent a `simple-expression` based on `WITH`. This subsection seems to be focused on defining exceptions, but it does not cover *consuming* those license exceptions. And “and exception … to which the exception applies” is really not clear; is it the left or right `WITH` operand? What happens for licenses/exceptions that are not in the SPDX License List? | ||
| * `spdx:licenseExceptionText`: Full text of the license exception. | ||
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| <!-- the following code block is not part of the preceding list --> | ||
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| <rdf:Description rdf:about="http://example.org#SPDXRef-ButIdDontWantToException"> | ||
| <rdfs:comment>This exception may be invalid in some jurisdictions.</rdfs:comment> | ||
| <rdfs:seeAlso>http://dilbert.com/strip/1997-01-15</rdfs:seeAlso> | ||
| <spdx:example>So this one time, I had a license exception…</spdx:example> | ||
| <spdx:licenseExceptionText> | ||
| A user of this software may decline to follow any subset of the terms of this license upon | ||
| finding any or all such terms unfavorable. | ||
| </spdx:licenseExceptionText> | ||
| <spdx:name>"But I Don't Want To" Exception</spdx:name> | ||
| <spdx:licenseExceptionId>SPDXRef-ButIdDontWantToException</spdx:licenseExceptionId> | ||
| <rdf:type rdf:resource="http://spdx.org/rdf/terms#LicenseException"/> | ||
| </rdf:Description> | ||
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| ### IV.3.3 Conjunctive `AND` Operator <a name="IV.3.3"></a> | ||
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| A [conjunctive license](#IV.2.3) can be expressed in XML via a `<spdx:ConjunctiveLicenseSet>` element, with an `spdx:member` property for each element in the conjunctive license. Two or more members are required. | ||
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| <spdx:ConjunctiveLicenseSet> | ||
| <spdx:member rdf:resource="http://spdx.org/licenses/GPL-2.0"/> | ||
| <spdx:ExtractedLicensingInfo rdf:about="http://example.org#LicenseRef-EternalSurrender"> | ||
| <spdx:extractedText> | ||
| In exchange for using this software, you agree to give its author all your worldly possessions. | ||
| You will not hold the author liable for all the damage this software will inevitably cause not only | ||
| You will not hold the author liable for all the damage this software will inevitably cause not only | ||
| to your person and property, but to the entire fabric of the cosmos. | ||
| </spdx:extractedText> | ||
| <spdx:licenseId>LicenseRef-EternalSurrender</spdx:licenseId> | ||
| </spdx:ExtractedLicensingInfo> | ||
| </spdx:ConjunctiveLicenseSet> | ||
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| A disjunctive license can be expressed in RDF via a `<spdx:DisjunctiveLicenseSet>` element, with an spdx:member property for each element in the disjunctive license. Two or more members are required. | ||
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| ### IV.3.4 Disjunctive `OR` Operator <a name="IV.3.4"></a> | ||
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| A [disjunctive license](#IV.2.4) can be expressed in XML via a `<spdx:DisjunctiveLicenseSet>` element, with an `spdx:member` property for each element in the disjunctive license. Two or more members are required. | ||
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| <spdx:DisjunctiveLicenseSet> | ||
| <spdx:member rdf:resource="http://spdx.org/licenses/GPL-2.0"/> | ||
| <spdx:member> | ||
| <spdx:ExtractedLicensingInfo rdf:about="http://example.org#LicenseRef-EternalSurrender"> | ||
| <spdx:extractedText> | ||
| In exchange for using this software, you agree to give its author all your worldly possessions. | ||
| You will not hold the author liable for all the damage this software will inevitably cause | ||
| You will not hold the author liable for all the damage this software will inevitably cause | ||
| not only to your person and property, but to the entire fabric of the cosmos. | ||
| </spdx:extractedText> | ||
| <spdx:licenseId>LicenseRef-EternalSurrender</spdx:licenseId> | ||
| </spdx:ExtractedLicensingInfo> | ||
| </spdx:member> | ||
| </spdx:DisjunctiveLicenseSet> | ||
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| A License Exception can be expressed in RDF via a \<spdx:LicenseException> element. This element has the following attributes: | ||
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| * Comment - An `rdfs:comment` element describing the nature of the exception. | ||
| * See Also (optional)- An `rdfs:seeAlso` element referencing external sources of information on the exception. | ||
| * Example - Text describing examples of this exception. | ||
| * Name - The full human readable name of the item. | ||
| * License Exception ID: The identifier of an exception in the SPDX License List to which the exception applies. | ||
| * License Exception Text: Full text of the license exception. | ||
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| <rdf:Description rdf:about="http://example.org#SPDXRef-ButIdDontWantToException"> | ||
| <rdfs:comment>This exception may be invalid in some jurisdictions.</rdfs:comment> | ||
| <rdfs:seeAlso>http://dilbert.com/strip/1997-01-15</rdfs:seeAlso> | ||
| <spdx:example>So this one time, I had a license exception…</spdx:example> | ||
| <spdx:licenseExceptionText> | ||
| A user of this software may decline to follow any subset of the terms of this license upon | ||
| finding any or all such terms unfavorable. | ||
| </spdx:licenseExceptionText> | ||
| <spdx:name>"But I Don't Want To" Exception</spdx:name> | ||
| <spdx:licenseExceptionId>SPDXRef-ButIdDontWantToException</spdx:licenseExceptionId> | ||
| <rdf:type rdf:resource="http://spdx.org/rdf/terms#LicenseException"/> | ||
| </rdf:Description> | ||
| [rdfs:comment]: https://www.w3.org/TR/2014/REC-rdf-schema-20140225/#ch_comment | ||
| [rdfs:seeAlso]: https://www.w3.org/TR/2014/REC-rdf-schema-20140225/#ch_seealso | ||
| [rfc5234]: http://tools.ietf.org/html/rfc5234 | ||
| [rfc5234-aB]: https://tools.ietf.org/html/rfc5234#appendix-B |