Method set_error

include/exec/sequence/merge_each.hpp:787–791

      template <class _Error>
      void set_error(_Error&& __error) noexcept
      {
        __op_->store_error(static_cast<_Error&&>(__error));
        __next_seq_op_->nested_sequence_break();
      }

      void set_stopped() noexcept
      {