anchor.diff:  
   usertype_variable_declaration   : (Automatic)? user_type (dimension)? list_of_variable_descriptions ;
   
  -string_declaration  :   String list_of_variable_descriptions ;
  +string_declaration  :   SVString list_of_variable_descriptions ;
   
   struct_declaration  :   struct_type list_of_variable_descriptions ;
   
  
positive.code:                 | Realtime
                  | Time
                  | Reg
                  | String
                  | bits_type
                  | user_type
                  ;
  
negative.code: Specify : 'specify' ;
  Specparam : 'specparam' ;
  Static : 'static' ;
  SVString : 'string' ;
  Strong0 : 'strong0' ;
  Strong1 : 'strong1' ;
  Struct : 'struct' ;
  