anchor.diff:  tuple
   %struct
   record_expr record_tuple record_field record_fields
  +map_expr map_tuple map_field map_fields map_key
   if_expr if_clause if_clauses case_expr cr_clause cr_clauses receive_expr
   fun_expr fun_clause fun_clauses atom_or_var integer_or_var
   try_expr try_catch try_clause try_clauses
  
positive.code: 
  %%struct -> atom tuple :
  %%	{struct,?line('$1'),element(3, '$1'),element(3, '$2')}.
  
  
  %% N.B. This is called from expr_700.
  
negative.code: 
  expr_600 -> prefix_op expr_700 :
  	?mkop1('$1', '$2').
  expr_600 -> map_expr : '$1'.
  expr_600 -> expr_700 : '$1'.
  
  expr_700 -> function_call : '$1'.
  