Core Cauterize components including: the schema compiler, the meta compiler, and the test infrastructure.
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   )     _____             _            _
  /((   /  __ \           | |          (_)
 (_))\  | /  \/ __ _ _   _| |_ ___ _ __ _ _______
 _)((_) | |    / _` | | | | __/ _ \ '__| |_  / _ \
 \ ^ /  | \__/\ (_| | |_| | ||  __/ |  | |/ /  __/
  \_/    \____/\__,_|\__,_|\__\___|_|  |_/___\___|


Cauterize is a data-description language suitable for hard-real-time systems and systems without dynamic memory allocation. It can be used instead of other data description languages like JSON, XML, or ProtocolBuffers.


Cauterize is first intended to serve the constraints of hard real-time embedded systems programming, but is still suitable for a variety of other situations. All Cauterize specifications have the following properties:

  • Encoded messages have a maximum and minimum size known at compile time
  • All types have a maximum referential depth
  • All types are neither directly nor indirectly recursive
  • All types are listed in a topographically sorted order (all types that depend on other types will follow those types in the specification output)
  • The specification has a version-hash that is based on all possible variation in the schema. This can be used to detect incidental (as opposed to adversarial) protocol drift with a very high probability.
  • All types have a type-hash based on the structure of that type and all types it depends on
  • All length, field, and type tags have their representation optimized to use as little space as possible while still maintaining a minimum alignment of 8 bits

Cauterize was first designed to work on a small embedded system that required all memory to be allocated ahead of time (no dynamic memory allocation) yet still needed to interface with a Ruby and C# environment. The prototype was a Ruby DSL that, when executed, would emit C, C#, and Ruby libraries that were all capable of reading and writing the schema described by the DSL.

This project is a successor to the original Ruby DSL prototype with a goals of being safer, being more complete, and including features making it easier to add new code generators beyond the original C, C# and Ruby generators.

In order to better frame the context at which Cauterize is targeted, here's a incomplete list of other tools that attempt to perform some or all of the functions Cauterize is capable of performing. If Cauterize is not right for your purposes, perhaps one of these tools is. These are listed alphabetically.

Schema, Specification, and Code Generation

Cauterize consists of several parts: a schema language for describing ordinary data, a compiler to translate a schema into an intermediate representation known as a specification, and code generators that translate specifications into encoders and decoders.

A schema is written by humans and it describes the semantic meaning in all types.

The specification is created by the cauterize compiler and it describes all inferable information from the schema in order to make the creation of code generators easier.

Code generators consume the specification and output a library capable of encoding and decoding data represented by the schema.


Cauterize should be suitable for hard-real-time systems without dynamic memory allocation. From this goal, we can extract the following more specific goals:

  • Must be achievable with static memory allocation - not all embedded systems support dynamic allocation.
  • Must be achievable in bounded execution time - hard-real-time systems must know how long each operation can possibly take.
  • Must support methods for detecting protocol drift early - embedded systems are often harder to update than desktop systems. They have longer deployment in more unusual conditions. Therefore, it is very important that the version of the messages being used by the embedded systems is detectable by its partner systems and that they be kept in sync.
  • Specifications must be precise in as many ways as possible - many embedded systems vary from standard desktop and server systems in unusual ways. These variations can can include things such as: the number of bits in a byte, the amount of memory available on the system, the representation of pointers, the endianness of the processor, and the format of floating point numbers.

After dealing with these points that enable Cauterize to be used in constrained systems, there are several other goals that make the quality of life for users better.

  • Should not preclude other systems - though embedded systems are a primary target, design choices for Cauterize should not preclude the use of Cauterize on systems such as mobile, desktop, and web development.
  • Ease of implementation - code generators should be able to represent the specification in idioms common in the target language. In C, this is structs, enumerations, and unions. In Ruby, this would likely be classes. Furthermore, code generators should not have to generate any overly-complicated structures to conform to a specification. When a Cauterize feature is proposed, it must be implementable in simple terms in a variety of languages.
  • Ease of verification - code generators are hard to validate for correctness. There should be some means of checking them automatically.
  • Simplicity - code generators should not be expected to perform complicated operations in order to emit code. Concepts should be simple in nature and have at least one obvious method for implementation.

Schema Language

The schema language uses parentheses to enclose each of its expressions. All expressions have an assigned order for all arguments.

Schema Name

The schema name is optional. If it's omitted, it will be set to "schema".

(name [name])

Names can consist of the following characters and must be enclosed in double quotations:


Schema Version

The schema version is optional. If it's omittied, it will be set to "0.0.0". The version may consist of the following characters in quotations:


The reason the name and version patterns are restrictive is to ease the burden on code generators. Some languages, such as Haskell and Ruby, have specific rules about capitalization. Without the restrictive name pattern Cauterize uses, code generators would have to do a lot more work to emit code that is readable and matches the target language's normal conventions.


Line comments are defined by using two ; characters in a row. Here's an example.

;; this is comment
(name "some name")
(version "0.0.1")

Primitive Types

There are several types that represent the foundation of the Cauterize types. These are the fundamental types available for creating your own more complex types. It is not possible to define new built-in types in a schema. All builtins referenced by a schema will have definitions in the output specification.

Unsigned Primitive Types

Unsigned values are encoded as little endian.

  • u8 - 8 bits wide
  • u16 - 16 bits wide
  • u32 - 32 bits wide
  • u64 - 64 bits wide

Signed Primitive Types

Signed values are encoded as two's complement little endian values.

  • s8 - 8 bits wide
  • s16 - 16 bits wide
  • s32 - 32 bits wide
  • s64 - 64 bits wide

Boolean Primitive Type

Booleans are encoded with a single byte. The only valid values are 0 and 1 where 0 represents false and 1 represents true.

  • bool - 8 bits wide

Floating Point Primitive Types

Floating point types are hard. Their definitions can be different (or missing entirely!) across CPU architectures. Therefore, Cauterize only defines the f32 and f64 types. These are the IEEE 754 single and double precision floating point values. The single precision value uses 32 bits while the double-precision value uses 64 bits. Both flavors are encoded in little endian.

Floating point types:

  • f32 - 32 bits wide, IEEE754 Single Precision
  • f64 - 64 bits wide, IEEE754 Double Precision


Cauterize provides several prototypes that act as templates out of which other types can be created.

All types must list a name. That name follows the following rule:



Synonyms are used to give one of the built-in types a new name. Their encoded representation is identical to that of the built-in value they wrap, but has a type that is distinct from the wrapped value.

(type [type name] synonym [built-in type name])

The following example defines the type age that has the same representation as a u8.

(type age synonym u8)


Ranges are used to encode an integer value between two other integer values. They are encoded in a word suitable for expressing all possible values in the range. That is, a range with less than 256 members will be encoded in a 8 bit word, a range with less than 65535 members will be encoded in a 16 bit word, and so on.

(type [type name] range [minimum value] [maximum value])

The following is an example of a range that only encodes the values from 1000 to 1010.

(type some_range range 1000 1010)


Arrays are fixed-length sequences of identially typed objects. These are to be used when the sequence only makes sense with a fixed number of elements.

(type [type name] array [element type] [array length])

Consider the following example that defines a type mac that encodes a Media Access Control address (which is always 6 bytes long).

(type mac array u8 6)


Vectors are variable-lengthed sequences of identically typed objects. These are to be used when a sequence of elements has a maximum length, but may contain fewer elements.

(type [type name] vector [target type] [maximum array length])

The following example defines a generic byte buffer with a maximum length of 4096 bytes.

(type byte_buffer_4k vector u8 4096)


Enumerations are types with a fixed set of named members. Members of an enumeration are assign integer values starting from 0. These values are assigned automatically by the Cauterize compiler. Enumerations are encoded in the smallest word necessary to express every value in the enumeration.

(type [type name] enumeration (values [space-separated list of identifiers])

The following is an enumeration encoding the days of the week:

(type days_of_week enumeration

Field Lists

Field lists cannot be defined on their own; they can only be used as the last parameter to a record, union, or combination expression (which are defined later in this document). Field lists are used to designate a set of (name/type) pairs.

Unions and combinations can use the empty keyword instead of the field keyword. Empty fields do not have any associated data.

Field lists are defined like this:

  (field [field name] [type]) ;; a field with some data
  (empty [field name])        ;; an empty field (just the tag)
  (field ...))

A type is not required. The behavior of a type-less field is dependent on the enclosing expression.


Records are a collection of named fields where each field has a distinct type.

(type [type name] record [field list])

An empty field in a record lacks any semantic meaning. It can neither be encoded or represented by code generators.

This is an example of a record describing a person's age, height, and whether or not they like cats:

(type person record (fields (field age u8)
                            (field height u8)
                            (field likes_cats bool)))


Unions encode a set of possible values and some associated sub-value. Like records, their schema entries specify a list of fields, but, unlike records, only one of those fields can represented in a union at any given time.

Unions in Cauterize are very similar to algebraic data types found in other languages such as OCaml, Haskell, and Rust.

(type [type name] union [field list])

An empty field in a union represents that a variant of the union is set. This has meaning even if there is no associated data. A union where all fields lack associated data behaves similarly to a C enumeration.

This example shows a request type for some key-value storage system. The system stores u64 values according to names that are up to 128 bytes long.

(type key_name vector key_name u8 128)
(type key_pair record (fields
                       (field name key_name)
                       (field value u64)))

(type request union (fields
                     (field get_key_count)
                     (field check_key_exists key_name)
                     (field get_key key_name)
                     (field erase_key key_name)
                     (field set_key key_pair))))

The get_key_count variant does not contain any associated data while the get_key variant specifies a type that encodes the name of the key to get. Note: the response type is not defined in this example.


Combinations, like records, are a collection of named fields where each field has a distinct type. The difference is that each field in the combination can either be present or not present in the encoded output.

(type [type name] combination [field list])

As an example, consider the following description of a type capable of storing changes in some observed values in a sensor rig:

An empty field in a Combination behaves like a boolean flag.

(type sensed combination (fields
                          (field ambient_temp u16)
                          (field ambient_light u16)
                          (field air_pressure u16)
                          (field position_x u32)
                          (field position_y u32)
                          (field position_z u32)))

If a sensor value hasn't changed since the last time the message was sent, the message is able to omit that reading since there isn't new information to share.

Specification Language

The specification language also uses parenthesis to enclose each of its expressions. All expressions have an assigned order for all their arguments. Each expression explains one type defined by the schema.

Specification-specific Expressions

There are several expressions that show up in specifications that do not show up in schemas. These expressions represent data inferred from the schema.

sha1 Expression

The sha1 expression represents a SHA1 hash. It is 40 hexadecimal characters long.

(sha1 77e8f0d33bd09411bbc2f94c839e0ccc34d55603)

depth Expression

The depth expression represents the maximum referential depth of a schema. This expression is used in the top level specification expression.

(depth 6)

type-width Expression

The type-width expression represents the minimum length of the prefix of each type hash needed for a unique value. It is only used in the specification expression.


The two hashes below have the first two bytes in common. The type-width would be 3 because a 3-byte prefix of each hash is unique.

(sha1 77e8f0d33bd09411bbc2f94c839e0ccc34d55603)
(sha1 77e878098602c275eb7a3408aff17e396220324d)

The type-width expression would show up in the specification as:

(type-width 3)

length-width Expression

The length-width expression represents the number of bytes suitable for representing the maximum encoded length of any type in the schema.

This width is always one of 1, 2, 4, or 8.


For a schema with a maximum length of 68: (length-width 1).

For a schema with a maximum length of 257: (length-width 2).

For a schema with a maximum length of 70,000: (length-width 4).

For a schema with a maximum length of 17,000,000: (length-width 4).

For a schema with a maximum length of 8,600,000,000: (length-width 8).

fixed-size Expression

The fixed-size expression represents a size in bytes. This expression is used in type specifications that have a fixed encoding size.

(fixed-size 8)

range-size Expression

The range-size expression represents a minimum and maximum size in bytes. This expression is used in type specifications that have a variable encoding size.

(range-size 22 16406)

length-repr Expression

The length-repr expression represents the type used to encode a vector's length. It only occurs in vector specifications.

(length-repr u8)

flags-repr Expression

The flags-repr expression represents the type used to encode a combination's flags. It only occurs in combination specifications.

(flags-repr u16)

tag-repr Expression

The tag-repr expression represents the type used to encode a union's type tag. It only occurs in union specifications.

specification Expression

All specification documents contain several top-level expressions.

(name [schema name])
(version [schema version])
(sha1 [a sha1 hash])
(range-size [minimum encoded size] [maximum encoded size])
(depth [maximum referential depth of the schema])
(type-width [type-tag width hint])
(length-width [length-tag width hint])

[[type expressions]]

synonym Specification Expression

All synonym expressions have the following layout:

(type [type name] synonym (sha1 ...) (fixed-size ...))

array Specification Expression

All array expressions have the following layout:

(type [type name] array (sha1 ...) (range-size ...)
  [array length] [element type])

vector Specification Expression

All vectors have the following layout:

(type [type name] vector (sha1 ...) (range-size ...)
  (length-repr ...)
  [vector max length] [element type])

fields and field Specification Expressions

Record, union, and combination types all include a list of fields. This list of fields is enclosed in a fields expression.

The fields expression has this form:

(fields [[field expression]])

field expressions come in two varieties. The first variety expresses a name for the field, a type the field references, and the index of the field.

(field [field name] [type name] [index])

The second variety expresses only a name for the field and the index of the field. These are known as "empty" fields.

(field [field name] [index])

record Specification Expression

All records have the following layout:

(type [type name] record (sha1 ...) (range-size ...)
  (fields ...))

union Specification Expression

All unions have the following layout:

(type [type name] record (sha1 ...) (range-size ...)
  (tag-repr [built-in type])
  (fields ...))

combination Specification Expression

All combinations have the following layout:

(type [type name] combination (sha1 ...) (range-size ...)
  (flags-repr [built-in type])
  (fields ...))

Binary Interpretation

It's possible, given a binary encoding of a Cauterize type and the specification document for the type's schema, to interpret the encoded bytes into the original structure.

Types have two general flavors: types with a decision to make and types with only a single interpretation path. The types that have interpretation decisions are: vector, union, and combination. All others (builtin, synonym, array, and record) only have a single path of interpretation.

Decision types all encode their decision variable in the binary stream. For vector this is a length tag. For union, this is a type tag. For combination, the tag encodes a series of flags representing which fields in the combination are present.

Tag information for types always comes before the type data itself. For example, a vector with a u8 tag for its length will encode that u8 as the very first byte.

Furthermore, the only types that actually contain data are the builtin types. All other types are constructed from builtin types or other user-defined types. With knowledge about tag position and where data is stored, we can begin to parse encoded binary strings.

Sample Schema

We'll use the following as our schema. There's nothing particularly interesting about it except that it uses all of the available prototypes. I've called this file binterp.caut.

(schema binterp
  (synonym syn_u32 u32)

  (array arr_u32 u32 4)

  (vector vec_u32 u32 4)

  (record rec_unsigned
      (field fu8 u8)
      (field fu16 u16)
      (field fu32 u32)
      (field fu64 u64)))

  (union union_unsigned
      (field fu8 u8)
      (field fu16 u16)
      (field fu32 u32)
      (field fu64 u64)))

  (combination comb_unsigned
      (field fu8 u8)
      (field fu16 u16)
      (field fu32 u32)
      (field fu64 u64))))

The following command converts this schema into a specification: cauterize --schema=binterp.caut --output=binterp.cautspec.

When we inspect the specification, we see the following:

(name "binterp")
(version "")
(sha1 ac9fda94cadb44d18af85d498f169609fd716efb)
(range-size 1 17) (depth 2) (type-width 1) (length-width 1)
(type vec_u32 vector
  (sha1 35832f3b7bd6dbeb8d3b5c92f73b2f06759d2e7a)
  (range-size 1 17)
  (length-repr u8)
  4 u32)
(type union_unsigned union
  (sha1 e59761d5c25294927e5026c278db565f7190b693)
  (range-size 2 9)
  (tag-repr u8)
    (field fu8 u8 0)
    (field fu16 u16 1)
    (field fu32 u32 2)
    (field fu64 u64 3)))
(type syn_u32 synonym
  (sha1 f180b823f00f965e1f0f68ba5c82400f2d9dd32a)
  (fixed-size 4)
(type rec_unsigned record
  (sha1 b58dd55deef9faf22ac07ced17cf6f87d1c95111)
  (range-size 15 15)
    (field fu8 u8 0)
    (field fu16 u16 1)
    (field fu32 u32 2)
    (field fu64 u64 3)))
(type comb_unsigned combination
  (sha1 d67b5d0a49e122140f418c12ad445ed013a52fc3)
  (range-size 1 16)
  (flags-repr u8)
    (field fu8 u8 0)
    (field fu16 u16 1)
    (field fu32 u32 2)
    (field fu64 u64 3)))
(type arr_u32 array
  (sha1 965f3610970341adb1132d27a668a4c94e9e3d57)
  (range-size 16 16)
  4 u32))

Using this document and the knowledge of what type we're trying to decode, we can decode any encoded message for a particular specification.

It's important to remember that an encoded type, on its own, does not contain enough information to identify it as that type. Two peers wanting to transcode the same binary stream must agree on what type is being exchanged ahead of time or have a method for identifying which type is being encoded on the wire. Cauterize generators should normally emit the standard message interface based off the type-width and length-width parameters in the specification.

In the following exercises, all encoded messages will be listed in hexadecimal.

Decoding a Primtive

The following is an encoded u64 type:


Decoding primitives is pretty simple. Each builtin type has a fixed size. To decode a primitive, read that many bytes from the encoded string as a little endian value of the proper type.

The above example is, therefore, the following 64-bit value: 0x000000000003752A.

Decoding an Array

Decoding arrays is more complex than decoding builtins, but not much more. Array types all have a specific length. When decoding an array, one has to look at the array's element type and decode as many of that type as the array's length expression specifies.

The following is an encoded arr_u32 type.


Let's take a look at the arr_u32 specification:

(type arr_u32 array
  (sha1 965f3610970341adb1132d27a668a4c94e9e3d57)
  (range-size 16 16)
  4 u32))

We know, from the specification, that an arr_u32 type has a length of 4 and its element type is u32. We know, from its specification, that each u32 is made up of 4 bytes. So, all we need to do is read 4 u32 types from the binary string.

Therefore, we know that our decoded array is the folling list of u32 values:

[ 0x00000F8C, 0x00000AA3, 0x00000DD3, 0x0000082C ]

Decoding a Record

Decoding a record is quite similar to decoding an array. Both types have a fixed number of types to decode. The major difference is that arrays decode a specific number of the same types while records decode a specific number of varrying types.

The following is an encoded rec_unsigned type.


Let's take a look at the rec_unsigned specification:

(type rec_unsigned record
  (sha1 b58dd55deef9faf22ac07ced17cf6f87d1c95111)
  (range-size 15 15)
    (field fu8 u8 0)
    (field fu16 u16 1)
    (field fu32 u32 2)
    (field fu64 u64 3)))

To decode a record, we only need to decode each of the record's fields in order until we've decoded all the fields. For rec_unsigned, this means that we start by decodin fu8 and finish by decoding fu64.

So, we end up with the following list of decoded values:

[ 0xFB, 0x0F5E, 0x0000080B, 0x00000000000085CE ]

Decoding a Vector

Decoding a vector is very similar to decoding an array. The only difference is that the number of elements to decode is goverend by a length tag rather than by the type. Vectors define a maximum number of elements to decode rather than a constant number of elements to decode.

The following is an example of an encoded vec_u32.


Let's take a look at the specification for a vec_u32 again.

(type vec_u32 vector
  (sha1 35832f3b7bd6dbeb8d3b5c92f73b2f06759d2e7a)
  (range-size 1 17)
  (length-repr u8)
  4 u32)

The specification for a vector has a length-repr expression. This tells us what type is used to encode the length of this vector. In this case, a u8 is used to encode the length. Decoding a single byte from our binary string yields a value of 0x02. Therefore, we know that our encoded vector contains two elements. Since our element type is u32, we know to decode two u32 values from the binary string. This yields a vector of length 2 with the following value:

[ 0x000005F8, 0x000003AA ]

Decoding a Union

The following is an encoded union_unsigned.

01 af 04

To interpret this, we can reference the union_unsigned definition in our specification.

(type union_unsigned union
  (sha1 e59761d5c25294927e5026c278db565f7190b693)
  (range-size 2 9)
  (tag-repr u8)
    (field fu8 u8 0)
    (field fu16 u16 1)
    (field fu32 u32 2)
    (field fu64 u64 3)))

This tells us a few things: first, the encoded size will be between 2 and 9 bytes long. Our message is 3 bytes long. Next up, it tells us that the union's tag will be represented as a u8. Finally, we see 4 fields are associated with the union. Each field has an associated index. The value of the tag must match one of these indices. The index that matches is the type that the union will contain.

In our encoded message, we see that our first byte is 01. We know that, since we're decoding a union, this tag will match one of the field indices. As it turns out, this index maps to the field fu16. This field is associated with the u16 type.

A u16 is a primitive type. This means that it is a type that contains an actual value. Furthermore, we know that that this type has a fixed-size of 2. To decode a u16 type, all we need to do is read out 2 bytes from the binary stream

The next two bytes are af 04. We know that all Cauterize primitives are expressed in little endian, so this yields the final hex value of 0x04AF, or 1199 in decimal.

At this point, there's nothing else to decode! We are out of bytes and the type union_unsigned needs no more bytes to be complete.

Therefore, we can say that our union_unsigned value wraps a u16 with the value of 1199.

Decoding a Combination

The following is an encoded comb_unsigned.


To interpret this, we can reference the comb_unsigned type in our specification.

(type comb_unsigned combination
  (sha1 d67b5d0a49e122140f418c12ad445ed013a52fc3)
  (range-size 1 16)
  (flags-repr u8)
    (field fu8 u8 0)
    (field fu16 u16 1)
    (field fu32 u32 2)
    (field fu64 u64 3)))

Combinations use a set of flags to indicate which fields in the message are encoded. The flags are always at the beginning of the message. We can look at the flags-repr expression in the combination specification to determine how wide the word used to represent the flags is. In the case of comb_unsigned, the flags are represented as a u8.

We can see, based on the flags-repr expression in comb_unsigned that, the flags in our current example are represented by the byte 0x03--the first byte of our message. Remember, if flags-repr was a differrent type, we'd use more than one byte for our flags.

To start decoding fields in our combination, we start with the first field, shift 1 to the left by the index of the field, and check whether or not the bit is set in our flags. If the bit is set, we can then decode that type out of the binary string. If the bit is not set, the field is skipped and we move on to the next one.

We can see that our example has bits (1 << 0) (bit index 0) and (1 << 1) (bit index 1) set. This means that our first two fields, fu8 and fu16 are present in the binary string. The first field has type u8 and the second field has type u16. The field fu8 deocdes as the value 0x2C and the field fu16 decodes as the value 0x06d5 (remember, all builtin types are little endian).

Our final type is, therefore, a comb_unsigned with the field fu8 set to the value 0x2C and the field fu16 set to the value 0x06d5. The fields fu32 and fu64 are not set in this encoded instance.

Message Interface

TODO: Write about me.

Answers to Obvious Questions

In this section, we'll try and justify a few of the obvious questions that come up when reading this document. Cauterize has some odd restrictions, but they are normally conscious decisions. If you have a question that you feel is obvious and isn't listed here, feel free to open an issue with the question.

Why isn't there a string type?

TODO: Answer this well. Strings are weird.

If your schema needs a string type, consider defining your own like this:

(schema string_example 1.0.0
  (vector utf8str8k u8 8192))

This is a vector of u8 values. Its string encoding isn't checked, but it's likely safe to assume that it should be valid UTF8 data based on the name.

Why don't unions support multiple types per alternative?

In languages like Haskell, Rust, and OCaml, we're able to define union types/sum types/algebraic alternative types that can contain multiple types per constructor.

This behavior has not yet been supported in Cauterize because it adds complexity to the C code that would need to be generated.

Take this hypothetical (but invalid) example:

(name "multi_data_union_example")
(version "1.0.0")

(type multi_type_field union
   (field a u8 u16 u32)))

In Haskell, we might be able to expand this union expression into the following type:

data MultiTypeField = A U8 U16 U32

In C, we'd need to do something like this:

struct multi_type_field {
  enum multi_type_field_tag {
  } _tag;

  union {
    struct {
      u8 ix0;
      u16 ix1;
      u32 ix2;
    } a;

There's no good way to express the names for different elements in the field. We could come up with something, but it's not an obvious or clear path forward. For this reason, we've chosen to omit multiple types per field in unions.


  • Exhaustive checking for hash-collisions... just in case
  • Hash algorithm selection
  • Add a "ranged" type that's similar to a scalar but only accepts n..m values in a builtin
  • Add ability to load schemas into other schemas
  • Add small expression language to schemas for computing sizes or sharing numeric information
  • Consider addition of generics to the schema
  • Expand the things synonyms can refer to.