Definition
signature_decl : declspec_list signature identifier ; | declspec_list signature identifier = exp ;
Purpose
A signature_decl declares a signature, or binds an existing one to a name. It is purely a name-manipulation mechanism only
Semantics
The initializer expression must be a combination of only variable_exps, field_exps, and call_exps. It must refer to a signature.
Exactly one sugnature_def may coexist with exactly one uninitialized sugnature_decl of the same name.
