anchor.diff:          )*
   
       rule assign_rvalue:
  -        ( action_statement
  -        | actionvalue_statement
  +        ( group_statement
           | case_statement
  -        | seq_statement
           | interface_declaration
           | TOKRULES [ COLON  VAR]
               ( rule_statement )*
  
positive.code:         | TOKPAR statement_list TOKENDPAR
          | return_statement
          | rule_statement
          | seq_statement
          | TOKWHILE paren_expression
              ( action_statement
              | group_statement
              | seq_statement
              | VAR SEMICOLON
              )
          | variable_declaration_or_call
  
      rule statement_list:
          ( single_statement)*
  
negative.code:     rule match_brace:
          LBRACE match_arg (COMMA match_arg)* RBRACE
  
      rule group_statement:
            TOKBEGIN statement_list TOKEND
          | TOKSEQ statement_list TOKENDSEQ
          | TOKACTIONSTATEMENT [ COLON  VAR] [ SEMICOLON ]
              statement_list
              TOKENDACTION [ COLON VAR]
          | TOKACTIONVALUESTATEMENT statement_list TOKENDACTIONVALUE
  
      rule single_statement:
            case_statement
          | TOKFOR LPAREN
                  for_decl_item (COMMA for_decl_item)* SEMICOLON
                  assign_rvalue SEMICOLON
  