anchor.diff:  add_expr -> add_expr add_op mult_expr :
   #binary_op{
     line=?line('$1'),
  -  type=op('$2'),
  +  type=?op('$2'),
     left='$1',
     right='$3'
   }.
  
positive.code: mult_expr -> mult_expr mult_op pow_expr :
  #binary_op{
    line=?line('$1'),
    type=op('$2'),
    left='$1',
    right='$3'
  }.
  
negative.code:       end;
      {error, {Line, _, {Message, Token}}, _} ->
        {error, {Line, lists:flatten(io_lib:format("~p ~p", [Message, Token]))}}
    end.