arfIndex -- Computes the Arf index of a numerical semigroup

Let A be the operator that sends a numerical semigroup S to the semigroup obtained by adjoining all elements of the form x+y-z with x \geq y \geq z in S (and x+y-z still bounded by the conductor region under consideration). Iterating A eventually stabilizes at the Arf closure of S; see arfClosure.

The function arfIndex returns the number of times A must be applied to reach this fixed point. In particular, arfIndex L is 0 exactly when the input semigroup is already Arf.