Appendix

Abstract

In Chapter One we stated without proof the fundamental identity

$${f_{xy}}\left( {a, \ b} \right) = {f_{yx}}\left( {a, \ b} \right)$$

(see (1.9)), and we have made heavy use of this identity in subsequent chapters. We did not prove it on its first appearance, not because the proof is inordinately difficult, but because we did not wish at that stage to hold up—or discourage —the reader who was concerned to understand the nature of the new concepts to which he was being introduced. In this brief appendix we supply the proof.