static <A> A getSingleton(L<A> l) {
  assertEquals(1, l(l));
  ret first(l);
}

static <A> A getSingleton(A[] l) {
  assertEquals(1, l(l));
  ret first(l);
}

ifclass IMultiSet
static <A> A getSingleton(IMultiSet<A> ms) {
  assertEquals(1, l(ms));
  ret first(ms);
}
endif