addable(C++26)

constexpr bool addable(
  const in-vector auto& in1,
  const in-vector auto& in2,
  const in-vector auto& out)
{
  return out.extent(0) == in1.extent(0) &&
         out.extent(0) == in2.extent(0);
} // (1)

constexpr bool addable(
  const in-matrix auto& in1,
  const in-matrix auto& in2,
  const in-matrix auto& out)
{
  return out.extent(0) == in1.extent(0) &&
         out.extent(1) == in1.extent(1) &&
         out.extent(0) == in2.extent(0) &&
         out.extent(1) == in2.extent(1);
} // (2)