anchor.diff:     ;
   
   expression_list
  -   : expression
  -   | expression_list ',' expression
  +   : expression+
      ;
   
   type
  
positive.code:    ;
  
  pick_tag_list
     : IDENTIFIER
     | pick_tag_list 'or' IDENTIFIER
     ;
  
  function_definition
  
negative.code:    ;
  
  top_declaration_sequence
     : top_declaration+
     ;
  
  top_declaration
  