static void assertSame(S msg default "", O a, O b) {
  if (a != b)
    fail(joinNemptiesWithColon(msg, a + " != " + b + " (" + identityHash(a) + "/" + identityHash(b) + ")");
}

static void assertSame(IF0<S> msg, O a, O b) {
  if (a != b)
    fail(joinNemptiesWithColon(msg!, a + " != " + b + " (" + identityHash(a) + "/" + identityHash(b) + ")");
}