Document inclusion in schemas

preserves-schema.md

@@ -114,10 +114,11 @@ A bundle of schema files is a directory tree containing `.prs` files.
 
 ### Clauses.
 
-    Clause            = (Version / EmbeddedTypeName / Definition) "."
+    Clause            = (Version / EmbeddedTypeName / Include / Definition) "."
 
     Version           = "version" "1"
     EmbeddedTypeName  = "embeddedType" ("#f" / Ref)
+    Include           = "include" string
     Definition        = symbol "=" (OrPattern / AndPattern / Pattern)
 
 **Version specification.** Mandatory. Names the version of the schema
@@ -130,6 +131,10 @@ declares that values parsed by the schema do not contain embedded
 definition in this or a neighbouring schema, it declares that embedded
 `Value`s must themselves conform to the named definition.
 
+**Include.** *Experimental.* Includes the contents of a neighbouring
+file as if it were textually inserted in place of this clause. The
+file path may be relative to the current file, or absolute.
+
 **Definition.** Each definition clause implicitly connects a pattern
 with a type name and a set of associated functions.
 
@@ -695,5 +700,8 @@ definitions for the metaschema.
    challenge in doing so is the way schemas are required to be
    *reversible* at present.
 
+ - Should `include` accept URLs, to be able to retrieve schema from
+   the web?
+
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 ## Notes