This exposition will have at its core a sequence of proofs designed to establish theorems. We shall distinguish among the theorems some which we shall call lemmas, propositions or corollaries. Traditionally, a lemma is a result of no intrinsic interest proved as a step towards the proof of a theorem; a proposition is a result of less inde- pendent importance than a theorem; and a corollary is an easy consequence of a theorem. The distinctions are of no formal significance, however, and we make use of them only as a way of providing signposts to the reader as to the relative importance of the results stated.