Abstract
Let F be a free group and R be a subgroup of F. It
is proved that
$$\begin{array}{l}
\Delta^{n}(F)\Delta^{m}(R)/ \Delta^{n+1}(F)\Delta^{m}(R),\\
\Delta^{n}(F)\Delta^{m}(R)/ \Delta^{n-1}(F)\Delta^{m+1}(R),\\
\Delta^{n}(F)\Delta^{m}(R)/ \Delta^{n}(F)\Delta^{m+1}(R)
\end{array}$$
are free-abelian. Explicit bases of first two and
complete descriptions of all these groups are also given.