// f must return a string
static S joinMap(O f, Iterable l) {
  ret join(map(f, l));
}

static S joinMap(Iterable l, O f) {
  ret joinMap(f, l);
}

static <A> S joinMap(Iterable<A> l, IF1<A, S> f) {
  ret joinMap(f, l);
}