Follow-up from "components for full 1-track and 2-track line implementations"

The following discussions from !1659 (merged) should be addressed:

• @graven started a discussion:

  This could also be written as:

  using f_ret_t = std::invoke_result_t< decltype(f), item_t >;

  (see here

• @graven started a discussion:

          using ret_sum_t = std::invoke_result_t< std::plus<>, f_ret_t, f_ret_t >