anchor.diff:  To resolve another grammar Ambiguouity, we no more declare bitStringValue. The parser will only return valueList. Extra checking 
   must be done during semantic checking.
   bitStringValue
  -	:	L_BRACKET identifier (COMMA identifier)* R_BRACKET		->^(BIT_STRING_VALUE identifier+)
  +	:	L_BRACE identifier (COMMA identifier)* R_BRACE		->^(BIT_STRING_VALUE identifier+)
   	;
   */	
   
  
positive.code: EXCEPT		:'EXCEPT';
  SET		:'SET';
  ASSIG_OP		: '::=';
  L_BRACKET	:	'{';	
  R_BRACKET	:	'}';	
  L_PAREN		:	'(';
  R_PAREN		:	')';
  COMMA		:	',';
  
negative.code: nULL:	NULL;
  
  bitStringType
  	:	a=BIT STRING (L_BRACE (bitStringItem (COMMA bitStringItem )* )? R_BRACE )?	-> ^(BIT_STRING_TYPE[$a] bitStringItem*)
  	;
  
  bitStringItem 
  