Can you more formally describe the product you are doing in standard doc strings `(/** */)` so they are picked up by scaladoc?

What it `[T]` in the parameter list?  Why is the column type of the second matrix `RowT` and not `Col2T` or something?

Can you explain the splitter?

I guess what you are doing is equivalent to a standard product followed by a hadamard (pointwise) product with a section matrix that is 0 or 1.  This selection matrix is block diagonal, and so you are only getting some of the i,j products of the row_i of one with col_j of the other.  Then, you are using this fact that you don't care about many of the vector products to remove them from consideration.

Is that right?