anchor.diff:  
       rule for_variable_assignment:
           term<<[]>> assign_opvalue
  -
  -    rule return_statement:
  -        TOKRETURN assign_rvalue SEMICOLON
  -
  -    rule case_statement:
  -        TOKCASE paren_expression
  -        ( TOKMATCHES
  -          (
  -              matches_clause
  -              COLON (QUESTION SEMICOLON | single_statement)
  -          )*
  -        | (
  -              (expr<<[]>> (COMMA expr<<[]>>)*
  -              | TOKDEFAULT
  -              )
  -              COLON (QUESTION SEMICOLON | single_statement)
  -          )*
  -        )
  -        TOKENDCASE
   
       rule declared_item:
           term<<[]>> [ assign_opvalue ]
  
positive.code:             [ provisos_clause ] SEMICOLON
              [
                  (   TOKENDMODULE [ COLON VAR]
                  |   ( module_contents_declaration | single_statement)+
                      TOKENDMODULE [ COLON VAR]
                  )
              ]
          | TOKTYPEDEF ( single_type_definition | NUM VAR SEMICOLON )
  
negative.code: 
      rule assign_rvalue:
          ( group_statement
          | interface_declaration
          | TOKRULES [ COLON  VAR]
              ( rule_statement )*
  