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

For normal subgoups $R$ and $S$ of a free group $F$, an identification of the subgroup $F\cap (1+\mathfrak r\mathfrak f\mathfrak s)$ is derived, and it is shown that the the quotient $\frac{F\cap (1+\mathfrak r\mathfrak f\mathfrak s)}{[R'\cap S',\, R\cap S][R'\cap S,\,R'\cap S][R\cap S',\, R\cap S']}$ is, in general, non-trivial.