anchor.diff:          )*
   
       rule assign_rvalue:
  -        ( action_statement
  -        | actionvalue_statement
  +        ( group_statement
           | case_statement
  -        | seq_statement
           | interface_declaration
           | TOKRULES [ COLON  VAR]
               ( rule_statement )*
  
positive.code: 
      rule return_statement:
          TOKRETURN assign_rvalue SEMICOLON
  
      rule group_statement:
          TOKBEGIN
          statement_list
          TOKEND
  
      rule seq_statement:
          TOKSEQ
          statement_list
          TOKENDSEQ
  
      rule action_statement:
          TOKACTIONSTATEMENT [ COLON  VAR] [ SEMICOLON ]
          statement_list
          TOKENDACTION [ COLON VAR]
  
      rule actionvalue_statement:
          TOKACTIONVALUESTATEMENT
          statement_list
          TOKENDACTIONVALUE
  
      rule case_statement:
          TOKCASE paren_expression
  
negative.code:     rule declared_item:
          term<<[]>> [ assign_opvalue ]
  
      rule for_decl_item:
          # datatype might not be present
          type_decl [ VAR ] assign_opvalue
  