 (The diligent reader will find that this presentation does not follow PM exactly. The example “the father of” based on a relation R expressing “x is the father of y” would make “the R of x” actually refer to the unique x which is the father of y, and so what has been explained above is appropriate to the converse of that relation, \(\relbR\). The practice of reading the argument of a relational function as the x and the value as the y is so well established that we have taken a liberty with the actual definitions in PM.)