He proceeds to show how to replace the "successor-predecessor" set { [ 0 ], [ ' ], [ - ] } with the "successor-equality" set { [ 0 ], [ ' ], [ ≠ ] }. And then he defines his "REPEAT" and shows that we can define any primitive recursive function by the "successor-repeat" set { [ 0 , [ ' ], } (where the range of the [ RPT cannot include itself. If it does, we get what is called the mu operator (see also mu recursive functions) (p. 213)):