From http://flash-gordon.me.uk/ansi.c.txt

Almost all contraints belongs to the compiler, except PERHAPS, I said PERHAPS
because this does NOT mean that MarpaX::Languages::C::AST is wrong. These
are constraints not in the grammar, as the few constraints already coded in
this package.

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.5.1 Storage-class specifiers

Syntax

          storage-class-specifier:
                  typedef
                  extern
                  static
                  auto
                  register

Constraints

   At most one storage-class specifier may be given in the declaration
specifiers in a declaration./48/
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3.5.2.1 Structure and union specifiers

Syntax

          struct-or-union-specifier:
                  struct-or-union identifier<opt> {  struct-declaration-list } 
                  struct-or-union identifier

          struct-or-union:
                  struct
                  union

          struct-declaration-list:
                  struct-declaration
                  struct-declaration-list struct-declaration

          struct-declaration:
                  specifier-qualifier-list struct-declarator-list ;

          specifier-qualifier-list:
                  type-specifier specifier-qualifier-list<opt>
                  type-qualifier specifier-qualifier-list<opt>

          struct-declarator-list:
                  struct-declarator
                  struct-declarator-list ,  struct-declarator

          struct-declarator:
                  declarator
                  declarator<opt> :  constant-expression

Constraints

   A structure or union shall not contain a member with incomplete or
function type.  Hence it shall not contain an instance of itself (but
may contain a pointer to an instance of itself).
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3.5.3 Type qualifiers

Syntax

          type-qualifier:
                  const
                  volatile

Constraints

   The same type qualifier shall not appear more than once in the same
specifier list or qualifier list, either directly or via one or more
typedef s.
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3.5.4.3 Function declarators (including prototypes)

Constraints

   A function declarator shall not specify a return type that is a
function type or an array type.

   The only storage-class specifier that shall occur in a parameter
declaration is register.

   An identifier list in a function declarator that is not part of a
function definition shall be empty.
--------------------------------------------------------------
3.5.7 Initialization

Syntax

          initializer:
                  assignment-expression
                  {  initializer-list } 
                  {  initializer-list , }

          initializer-list:
                  initializer
                  initializer-list ,  initializer

Constraints

   There shall be no more initializers in an initializer list than
there are objects to be initialized.
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3.6.1 Labeled statements

Syntax

          labeled-statement:
                  identifier :  statement
                  case  constant-expression :  statement
                  default :  statement

Constraints

   A case or default label shall appear only in a switch statement.
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3.6.6.1 The goto statement

Constraints

   The identifier in a goto statement shall name a label located
somewhere in the current function.
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3.6.6.2 The continue statement

Constraints

   A continue statement shall appear only in or as a loop body.  
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3.6.6.3 The break statement

Constraints

   A break statement shall appear only in or as a switch body or loop body.  
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3.6.6.4 The return statement

Constraints

   A return statement with an expression shall not appear in a
function whose return type is void .
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3.7 EXTERNAL DEFINITIONS

Syntax

          translation-unit:
                  external-declaration
                  translation-unit external-declaration

          external-declaration:
                  function-definition
                  declaration

Constraints

   The storage-class specifiers auto and register shall not appear in
the declaration specifiers in an external declaration.
--------------------------------------------------------------
3.7.1 Function definitions

Syntax

          function-definition:
                  declaration-specifiers<opt> declarator
                            declaration-list<opt> compound-statement

Constraints

   The identifier declared in a function definition (which is the name
of the function) shall have a function type, as specified by the
declarator portion of the function definition./70/