message: das.cli: fix removal of used arguments
  Should have been part of
diff: @@ -475,7 +475,8 @@ namespace elle
  {
  ELLE_TRACE("use next positional value: %s", *it);
  this->_values.emplace_back(std::move(*it));
  - it = this->_args.erase(it);
  + *it = nothing();
  + ++it;
  }
  else
  return this->missing<I>();
  