std::linalg::possibly-addable(C++26)

namespace std::linalg {
  template<in-vector In1, in-vector In2, in-vector Out>
  constexpr bool possibly-addable()
  {
    return compatible-static-extents<Out, In1>(0, 0) &&
           compatible-static-extents<Out, In2>(0, 0) &&
           compatible-static-extents<In1, In2>(0, 0);
  }   // (1)

  template<in-matrix In1, in-matrix In2, in-matrix Out>
  constexpr bool possibly-addable()
  {
    return compatible-static-extents<Out, In1>(0, 0) &&
           compatible-static-extents<Out, In1>(1, 1) &&
           compatible-static-extents<Out, In2>(0, 0) &&
           compatible-static-extents<Out, In2>(1, 1) &&
           compatible-static-extents<In1, In2>(0, 0) &&
           compatible-static-extents<In1, In2>(1, 1);
  }   // (2)
}