__
Sum of vectors
and
\ref{_vproduct_} produces the product of two vectors:
$V_k = A_k B_k$.
\ref{_vproduct} is its helper function for recursion.
\ref{_mproduct_} produces the product of two matrices:
$M_{ij} = A_{ij} B_{ij}$.
\ref{_mproduct} is its helper function for recursion.
\ref{_vproduct_} is used to reduce the problem to the product of
and array of~vectors.