diff --git a/gaps/GAP-33/DRAFT.md b/gaps/GAP-33/DRAFT.md
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+# Set Extensions for Type System Documents
+
+This document specifics an alternative to the [Type System Document](https://spec.graphql.org/draft/#sec-Type-System) for defining a set-compatible version of the type system of a GraphQL Schema.
+
+It is common to need to treat schema documents as sets: we may want
+to merge two documents to create a "Composite Schema", or you might
+want to know the intersection between documents to build
+products that work against all the provided GraphQL Services.
+
+Any grammar not defined, but used, in this document is a reference
+to the grammar defined in [the GraphQL Specification's Grammar](https://spec.graphql.org/draft/#sec-Appendix-Grammar-Summary).
+
+The key change to the grammar: **every location in the schema that can have a directive now can also be an Extension**.
+
+To put it another way, if a location in the Schema can be defined by a
+[Schema Coordinate](https://spec.graphql.org/draft/#sec-Schema-Coordinates), then it can exist in the Set Type System in an `extend` representation. Additionally, the root `schema` has an `extend` representation, because it can apply set operations.
+
+# Set Type System
+
+SetTypeSystemDocument : SetTypeSystemDefinitionOrExtension+
+
+SetTypeSystemDefinitionOrExtension :
+
+- SetTypeSystemDefinition
+- SetTypeSystemExtension
+
+SetTypeSystemDefinition :
+
+- SetSchemaDefinition
+- SetTypeDefinition
+- SetDirectiveDefinition
+
+SetTypeSystemExtension :
+
+- SetSchemaExtension
+- SetTypeExtension
+- SetDirectiveExtension
+
+## Schema
+
+SetSchemaDefinition : Description? schema Directives[Const]? {
+RootOperationTypeDefinition+ }
+
+RootOperationTypeDefinition : OperationType : NamedType
+
+### Schema Extension
+
+SetSchemaExtension :
+
+- extend schema Directives[Const]? { RootOperationTypeDefinition+ }
+- extend schema Directives[Const] [lookahead != `{`]
+
+SetSchemaDefinition and SetSchemaExtension are grammatically identical to the [Schema](https://spec.graphql.org/draft/#sec-Schema) in the GraphQL Specification.
+
+## Types
+
+SetTypeDefinition :
+
+- SetScalarTypeDefinition
+- SetObjectTypeDefinition
+- SetInterfaceTypeDefinition
+- SetUnionTypeDefinition
+- SetEnumTypeDefinition
+- SetInputObjectTypeDefinition
+
+### Type Extensions
+
+SetTypeExtension :
+
+- SetScalarTypeExtension
+- SetObjectTypeExtension
+- SetInterfaceTypeExtension
+- SetUnionTypeExtension
+- SetEnumTypeExtension
+- SetInputObjectTypeExtension
+
+## Scalars
+
+SetScalarTypeDefinition : Description? scalar Name Directives[Const]?
+
+SetScalarTypeDefinition and SetScalarTypeExtension are gramatically identical to the [Scalars](https://spec.graphql.org/draft/#sec-Scalars) Specification.
+
+### Scalar Extensions
+
+SetScalarTypeExtension :
+
+- extend scalar Name Directives[Const]
+
+## Objects
+
+SetObjectTypeDefinition :
+
+- Description? type Name ImplementsInterfaces? Directives[Const]?
+  SetFieldsDefinitionOrExtension
+- Description? type Name ImplementsInterfaces? Directives[Const]? [lookahead !=
+  `{`]
+
+ImplementsInterfaces :
+
+- ImplementsInterfaces & NamedType
+- implements `&`? NamedType
+
+SetFieldsDefinitionOrExtension : { SetFieldDefinitionOrExtension+ }
+
+### Object Extensions
+
+SetObjectTypeExtension :
+
+- extend type Name ImplementsInterfaces? Directives[Const]? SetFieldsDefinitionOrExtension
+- extend type Name ImplementsInterfaces? Directives[Const] [lookahead != `{`]
+- extend type Name ImplementsInterfaces [lookahead != `{`]
+
+## Fields
+
+SetFieldDefinitionOrExtension:
+
+- SetFieldDefinition
+- SetFieldExtension
+
+SetFieldDefinition :
+- Description? Name SetArgumentsDefinitionOrExtension? : Type Directives[Const]?
+- Description? Name SetArgumentsDefinitionOrExtension [lookahead != `:`] Directives[Const]?
+- Description? Name [lookahead != `:`] Directives[Const]?
+
+### Field Extensions
+
+SetFieldExtension :
+- extend Name SetArgumentsDefinitionOrExtension? : Type Directives[Const]?
+- extend Name SetArgumentsDefinitionOrExtension [lookahead != `:`] Directives[Const]?
+- extend Name [lookahead != `:`] Directives[Const]?
+
+It is possible for the type definition of a field to only be present in that field's extension. Likewise, we need to be able to extend a field (like deprecating it)
+with a guarantee that the field's type will not change through the extension.
+
+Field extensions are different from fields defined by a type extension. The below is valid syntax:
+
+```graphql field-extension
+extend type Person {
+  extend age @deprecated
+  name
+}
+
+type Business {
+  extend name @deprecated
+}
+```
+To become a valid GraphQL Spec document, the above would need to
+be `union`'d with a document like:
+
+```graphql field-extension-merge
+type Person {
+  age: Int
+  extend name: String @deprecated
+}
+
+extend type Business {
+  name: String
+}
+```
+
+resulting in:
+
+```graphql field-extension-union
+type Person {
+  age: Int @deprecated
+  name: String @deprecated
+}
+
+type Business {
+  name: String @deprecated
+}
+```
+which is now a valid GraphQL Type System Document.
+
+The syntax of allowing field definitions without a type definition
+enables tooling, such as diff tools, to have a valid *syntax* for
+how a schema is evolving, even if the *semantic meaning* of the syntax
+would not produce a valid GraphQL Schema.
+
+### Field Arguments
+
+SetArgumentsDefinition : ( SetInputValueDefinitionOrExtension+ )
+
+SetInputValueDefinitionOrExtension :
+- SetInputValueDefinition
+- SetInputValueExtension
+
+### Input Values
+
+SetInputValueDefinition :
+
+- Description? Name : Type DefaultValue? Directives[Const]?
+- Description? Name [lookahead != `:`] DefaultValue? Directives[Const]?
+
+### Input Value Extensions
+
+SetInputValueExtension :
+
+- extend Name : Type Directives[Const]?
+- extend Name [lookahead != `:`] Directives[Const]?
+
+Similar to Field Definitions and Extensions, Input Value Definitions and Extensions cannot may elide the input value's type.
+
+Example:
+```graphql argument-extension
+type Person {
+  name(short): String
+  extend age(minimum: Int)
+}
+```
+union
+```graphql argument-extension-union
+type Person {
+  extend name(extend short: Boolean)
+  age(extend minimum @deprecated): Int
+}
+```
+results in:
+```
+type Person {
+  name(short: Boolean): String
+  age(minimum @deprecated): Int
+}
+```
+
+SetInterfaceTypeDefinition :
+
+- Description? interface Name ImplementsInterfaces? Directives[Const]?
+  SetFieldsDefinitionOrExtension
+- Description? interface Name ImplementsInterfaces? Directives[Const]?
+  [lookahead != `{`]
+
+SetInterfaceTypeExtension :
+
+- extend interface Name ImplementsInterfaces? Directives[Const]?
+  SetFieldsDefinitionOrExtension
+- extend interface Name ImplementsInterfaces? Directives[Const] [lookahead !=
+  `{`]
+- extend interface Name ImplementsInterfaces [lookahead != `{`]
+
+## Unions
+
+SetUnionTypeDefinition : Description? union Name Directives[Const]?
+UnionMemberTypes?
+
+UnionMemberTypes :
+
+- UnionMemberTypes | NamedType
+- = `|`? NamedType
+
+### Union Extensions
+
+SetUnionTypeExtension :
+
+- extend union Name Directives[Const]? UnionMemberTypes
+- extend union Name Directives[Const]
+
+SetUntionTypeExtension is gramatically identical to the [Unions](https://spec.graphql.org/draft/#sec-Unions) in the GraphQL Specification.
+
+## Enums
+
+SetEnumTypeDefinition :
+
+- Description? enum Name Directives[Const]? SetEnumValuesDefinitionOrExtension
+- Description? enum Name Directives[Const]? [lookahead != `{`]
+
+### Enum Values
+
+SetEnumValuesDefinitionOrExtension : { SetEnumValueDefinitionOrExtension+ }
+
+SetEnumValueDefinitionOrExtension :
+- SetEnumValueDefinition
+- SetEnumValueExtension
+
+SetEnumValueDefinition : Description? EnumValue Directives[Const]?
+SetEnumValueExtension : extend EnumValue Directives[Const]?
+
+### Enum Extensions
+
+SetEnumTypeExtension :
+
+- extend enum Name Directives[Const]? SetEnumValuesDefinitionOrExtension
+- extend enum Name Directives[Const] [lookahead != `{`]
+
+## Input Objects
+
+InputObjectTypeDefinition :
+
+- Description? input Name Directives[Const]? SetInputFieldsDefinitionOrExtension
+- Description? input Name Directives[Const]? [lookahead != `{`]
+
+SetInputFieldsDefinitionOrExtension : { SetInputValueDefinitionOrExtension+ }
+
+## Input Object Extensions
+
+InputObjectTypeExtension :
+
+- extend input Name Directives[Const]? SetInputFieldsDefinitionOrExtension
+- extend input Name Directives[Const] [lookahead != `{`]
+
+## Directives
+
+NOTE: we are assuming that directives on directive definitions, (https://github.com/graphql/graphql-spec/pull/1206) is in the spec at this point.
+
+SetDirectiveDefinition : Description? directive @ Name SetArgumentsDefinitionOrExtension? Directives[Const]?
+`repeatable`? on DirectiveLocations
+
+DirectiveLocations :
+
+- DirectiveLocations | DirectiveLocation
+- `|`? DirectiveLocation
+
+DirectiveLocation :
+
+- ExecutableDirectiveLocation
+- TypeSystemDirectiveLocation
+
+ExecutableDirectiveLocation : one of
+
+- `QUERY`
+- `MUTATION`
+- `SUBSCRIPTION`
+- `FIELD`
+- `FRAGMENT_DEFINITION`
+- `FRAGMENT_SPREAD`
+- `INLINE_FRAGMENT`
+- `VARIABLE_DEFINITION`
+
+TypeSystemDirectiveLocation : one of
+
+- `SCHEMA`
+- `SCALAR`
+- `OBJECT`
+- `FIELD_DEFINITION`
+- `ARGUMENT_DEFINITION`
+- `INTERFACE`
+- `UNION`
+- `ENUM`
+- `ENUM_VALUE`
+- `INPUT_OBJECT`
+- `INPUT_FIELD_DEFINITION`
+- `DIRECTIVE_DEFINITION`
+
+### Directive Extensions
+
+SetDirectiveExtension : extend directive @ Name SetArgumentsDefinitionOrExtension? Directives[Const]
+
+# Set Operations
+
+With the provided syntax, we can create set operations
+- `union`, or ∪⁠ (also known as **merge** or **set addition**). `union` is associative and commutative, much like addition.
+- `intersect`, or ∩.
+- `exclude`, or \, or - (also known as **difference** or **set subtraction**). `exclude` is neither associative nor commutative, much like subtraction.
+
+## Union
+
+```graphql
+type A implements X {
+  field(arg: Int): String
+}
+```
+`union`
+```graphql
+type A @directive {
+  field(arg: Int!): String!
+}
+```
+will result in:
+```
+type A implements X @directive {
+  field(arg: Int!): String
+}
+```
+
+If a union could not possibly be commutative, i.e. the *order* of set union would affect the result,
+then the `union` operation should return a `UnionError`. The type of `A.field` below cannot be  determined.
+```
+type A {
+  field: Int!
+}
+```
+`union`
+```
+type A {
+  field: String!
+}
+```
+
+
+## Exclude
+
+## Intersect
+
+`A intersect B = A exclude (A exclude B)`.
+
+`intersect` is defined by the application of `exclude`.
+
+# Appendix: Grammar Summary
+
+SetTypeSystemExtensionDocument : SetTypeSystemDefinitionOrExtension+
+
+SetTypeSystemDefinitionOrExtension :
+
+- SetTypeSystemDefinition
+- SetTypeSystemExtension
+
+SetTypeSystemExtension :
+
+- SetSchemaExtension
+- SetTypeExtension
+
+SetSchemaDefinition : Description? schema Directives[Const]? {
+RootOperationTypeDefinition+ }
+
+SetSchemaExtension :
+
+- extend schema Directives[Const]? { RootOperationTypeDefinition+ }
+- extend schema Directives[Const] [lookahead != `{`]
+
+RootOperationTypeDefinition : OperationType : NamedType
+
+SetTypeDefinition :
+
+- SetScalarTypeDefinition
+- SetObjectTypeDefinition
+- SetInterfaceTypeDefinition
+- SetUnionTypeDefinition
+- SetEnumTypeDefinition
+- SetInputObjectTypeDefinition
+
+SetTypeExtension :
+
+- SetScalarTypeExtension
+- SetObjectTypeExtension
+- SetInterfaceTypeExtension
+- SetUnionTypeExtension
+- SetEnumTypeExtension
+- SetInputObjectTypeExtension
+
+SetScalarTypeDefinition : Description? scalar Name Directives[Const]?
+
+SetScalarTypeExtension :
+
+- extend scalar Name Directives[Const]
+
+SetObjectTypeDefinition :
+
+- Description? type Name ImplementsInterfaces? Directives[Const]? SetFieldsDefinitionOrExtension
+- Description? type Name ImplementsInterfaces? Directives[Const]? [lookahead != `{`]
+
+SetObjectTypeExtension :
+
+- extend type Name ImplementsInterfaces? Directives[Const]? SetFieldsDefinitionOrExtension
+- extend type Name ImplementsInterfaces? Directives[Const] [lookahead != `{`]
+- extend type Name ImplementsInterfaces [lookahead != `{`]
+
+ImplementsInterfaces :
+
+- ImplementsInterfaces & NamedType
+- implements `&`? NamedType
+
+SetFieldsDefinitionOrExtension : { SetFieldDefinitionOrExtension+ }
+
+SetFieldDefinitionOrExtension :
+- SetFieldDefinition
+- SetFieldExtension
+
+SetFieldDefinition :
+- Description? Name SetArgumentsDefinitionOrExtension? : Type Directives[Const]?
+- Description? Name SetArgumentsDefinitionOrExtension [lookahead != `:`] Directives[Const]?
+- Description? Name [lookahead != `:`] Directives[Const]?
+
+SetFieldExtension :
+- extend Name SetArgumentsDefinitionOrExtension? : Type Directives[Const]?
+- extend Name SetArgumentsDefinitionOrExtension [lookahead != `:`] Directives[Const]?
+- extend Name [lookahead != `:`] Directives[Const]?
+
+SetArgumentsDefinitionOrExtension : ( SetInputValueDefinitionOrExtension+ )
+
+SetInputValueDefinitionOrExtension :
+- SetInputValueDefinition
+- SetInputValueExtension
+
+SetInputValueDefinition :
+- Description? Name : Type DefaultValue? Directives[Const]?
+- Description? Name [lookahead != `:`] Directives[Const]?
+
+SetInputValueExtension :
+- extend Name : Type DefaultValue? Directives[Const]?
+- extend Name [lookahead != `:`] Directives[Const]?
+
+SetInterfaceTypeDefinition :
+
+- Description? interface Name ImplementsInterfaces? Directives[Const]? SetFieldsDefinitionOrExtension
+- Description? interface Name ImplementsInterfaces? Directives[Const]? [lookahead != `{`]
+
+SetInterfaceTypeExtension :
+
+- extend interface Name ImplementsInterfaces? Directives[Const]? SetFieldsDefinitionOrExtension
+- extend interface Name ImplementsInterfaces? Directives[Const] [lookahead != `{`]
+- extend interface Name ImplementsInterfaces [lookahead != `{`]
+
+SetUnionTypeDefinition : Description? union Name Directives[Const]?
+UnionMemberTypes?
+
+UnionMemberTypes :
+
+- UnionMemberTypes | NamedType
+- = `|`? NamedType
+
+SetUnionTypeExtension :
+
+- extend union Name Directives[Const]? UnionMemberTypes
+- extend union Name Directives[Const]
+
+SetEnumTypeDefinition :
+
+- Description? enum Name Directives[Const]? SetEnumValuesDefinitionsOrExtension
+- Description? enum Name Directives[Const]? [lookahead != `{`]
+
+SetEnumValuesDefinitionOrExtension : { SetEnumValueDefinitionOrExtension+ }
+
+SetEnumValueDefinitionOrExtension:
+- SetEnumValueDefinition
+- SetEnumValueExtension
+
+SetEnumValueDefinition : Description? EnumValue Directives[Const]?
+
+SetEnumValueExtension : extend EnumValue Directives[Const]?
+
+SetEnumTypeExtension :
+
+- extend enum Name Directives[Const]? SetEnumValuesDefinitionsOrExtension
+- extend enum Name Directives[Const] [lookahead != `{`]
+
+SetInputObjectTypeDefinition :
+
+- Description? input Name Directives[Const]? SetInputFieldsDefinition
+- Description? input Name Directives[Const]? [lookahead != `{`]
+
+SetInputFieldsDefinition : { SetInputFieldsDefinitionOrExtension+ }
+
+SetInputObjectTypeExtension :
+
+- extend input Name Directives[Const]? SetInputFieldsDefinition
+- extend input Name Directives[Const] [lookahead != `{`]
+
+SetDirectiveDefinition : Description? directive @ Name SetArgumentsDefinitionOrExtension?
+`repeatable`? on DirectiveLocations
+
+DirectiveLocations :
+
+- DirectiveLocations | DirectiveLocation
+- `|`? DirectiveLocation
+
+DirectiveLocation :
+
+- ExecutableDirectiveLocation
+- TypeSystemDirectiveLocation
+
+ExecutableDirectiveLocation : one of
+
+- `QUERY`
+- `MUTATION`
+- `SUBSCRIPTION`
+- `FIELD`
+- `FRAGMENT_DEFINITION`
+- `FRAGMENT_SPREAD`
+- `INLINE_FRAGMENT`
+- `VARIABLE_DEFINITION`
+
+TypeSystemDirectiveLocation : one of
+
+- `SCHEMA`
+- `SCALAR`
+- `OBJECT`
+- `FIELD_DEFINITION`
+- `ARGUMENT_DEFINITION`
+- `INTERFACE`
+- `UNION`
+- `ENUM`
+- `ENUM_VALUE`
+- `INPUT_OBJECT`
+- `INPUT_FIELD_DEFINITION`
+- `DIRECTIVE_DEFINITION`
+
+SetDirectiveExtension : extend directive @ Name SetArgumentsDefinitionOrExtension? Directives[Const]
diff --git a/gaps/GAP-33/README.md b/gaps/GAP-33/README.md
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+# GAP-33: GraphQL Schema Definition Language - Set Extensions
+
+## Overview
+
+This proposal adds syntax to the [GraphQL Type System](https://spec.graphql.org/draft/#sec-Type-System)'s IDL
+(sometimes called the "Schema Definition Language" or SDL) to allow for a *syntax* representation
+that support set *semantics*. This GAP does not yet define the *behavior* of set operations.
+
+Additionally, a Document using these set-extended semantics is *not* considered a
+GraphQL Type System Document, unless it would parse, validly, using the Spec standard Type System syntax.
+
+This is a potential alternative to https://github.com/graphql/gaps/pull/4.
+
+## Motivation
+
+It is common for tooling working with GraphQL to need to compose sets. This can be defined as a "union" operation.
+For instance, composite schemas need a merge operation, as defined in the [Composite Schemas Spec](https://graphql.github.io/composite-schemas-spec/draft/#sec-Merge).
+
+Another need is to partially *intersect* schemas. For instance, if we want to know the schema that
+is common across two different base schemas, an intersection lets us build products that will work against either.
+
+Another common requirement we've run into is determining whether a schema that an older client builds against
+is a *true subset* of the current server schema. We can determine this by using an *exclude* operation.
+
+However, the Type System Document syntax grammar does not support all of these operations well. For instance,
+how do you represent
+```graphql
+type A {
+  a(arg: Int): Int
+}
+
+type B {
+  b: Int @something
+}
+```
+**exclude**
+```graphql
+type A {
+  a: Int
+}
+
+extend type B {
+  b: Int
+}
+```
+
+The closest we could get in the current Spec syntax is:
+```graphql
+extend type A {
+  a(arg: Int): Int
+}
+
+type B {
+  b: Int @something
+}
+```
+Notice that we *do* have syntax, via the `extend` keyword, to showcase that `type A` is excluded from the exclude-set.
+But we don't have any syntax to show that `A.a` is excluded but `A.a(arg:)` is not.
+
+Likewise we can show that `type B` is *not* excluded, but we have no way of showing that the field, `B.b`, IS excluded, but the directive, `@something`, on `B.b` is NOT excluded.
diff --git a/gaps/GAP-33/metadata.yml b/gaps/GAP-33/metadata.yml
new file mode 100644
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--- /dev/null
+++ b/gaps/GAP-33/metadata.yml
@@ -0,0 +1,12 @@
+id: 33
+title: Set Extensions for Type System Documents
+summary: >
+  An auxiliary syntax extending the Type System Document syntax,
+  unlocking set-like operations (union, exclude, intersect) on Schema IDL.
+status: proposal
+authors:
+  - name: "Matt Mahoney"
+    email: "mahoney.mattj@gmail.com"
+    githubUsername: "@mjmahone"
+sponsor: "@magicmark"
+discussion: "https://github.com/graphql/gaps/pull/33"