Parabolic congruence subgroups in linear groups

Parabolic subgroups are described for the full and special linear groups over $a$ commutative ring $R$ which contain a principal congruence level a, where a is an ideal of $R$ such that $R/a$ is semilocal. It is assumed that $R$ is generated additively by its invertible elements and that the ring identity can be expressed as a sum of two invertible elements.