message: copy parameter number on copying parameter expression
diff: @@ -17,6 +17,7 @@ string ParameterExpression::ToString() const {
  unique_ptr<ParsedExpression> ParameterExpression::Copy() const {
  auto copy = make_unique<ParameterExpression>();
  + copy->parameter_nr = parameter_nr;
  copy->CopyProperties(*this);
  return move(copy);
  }
  