The subgroup determined by a certain ideal in a free group ring

For normal subgroups R and S of a free group F  , an identification of the subgroup F∩(1+rfs)F∩(1+rfs) is derived, and it is shown that the quotientF∩(1+rfs)[R′∩S′,R∩S][R′∩S,R′∩S][R∩S′,R∩S′] is, in general, non-trivial.