
  [;1m-spec partition(SetFun, Set) -> Partition[0m
  [;1m                   when[0m
  [;1m                       SetFun :: set_fun(),[0m
  [;1m                       Partition :: a_set(),[0m
  [;1m                       Set :: a_set().[0m

  Returns the partition of [;;4mSet[0m such that two elements are
  considered equal if the results of applying [;;4mSetFun[0m are equal.

    1> Ss = sofs:from_term([[a],[b],[c,d],[e,f]]),
    SetFun = fun(S) -> sofs:from_term(sofs:no_elements(S)) end,
    P = sofs:partition(SetFun, Ss),
    sofs:to_external(P).
    [[[a],[b]],[[c,d],[e,f]]]
