Extreme care must be taken in specifying required combinations to ensure that there are no "loops" in the specifications. A "loop" occurs when, through a series of required combination specifications, a word requires itself. The simplest case of a loop is:
A B B A
whereby word A requires word B, which in turn requires word A. Such a specification makes no sense. If words A and B must always appear together, why are they encoded as separate words? A more complex case of a loop occurs in the following specification:
A B B C C A
whereby word A requires word B, which in turn requires word C, which in turn requires word A.