In the context of this invention, ???heterocyclyl??? or ???heterocyclic ring system??? is a cyclic compound containing at least one heteroatom such as N, O, or S. A ???mixed heterocycle??? is a cyclic compound containing at least two heteroatoms such as N, O or S. A ???heteroaryl??? compound is a heterocycle containing at least one heteroatom such as N, O or S and is not fully saturated, e.g., is