The Categorize(f, L, opts) function categorizes the list L into lists  such that f(Mi[1], j, opts) returns true for each element j in the ith list Mi. The procedure f must return a value of type truefalse.

If f defines an equivalence relation, then the result is a partition of L into equivalence classes.

If f can be written in the form (x,y) -> g(x) = g(y), then the call ListTools[Classify](g, L, opts) is more efficient, and equivalent except in the form of the output (Classify returns a table of sets).