static void assertEqualsVerbose(O x, O y) {
  assertEqualsVerbose(null, x, y);
}

static void assertEqualsVerbose(S msg, O x, O y) {
  if (!(x == null ? y == null : x.equals(y)))
    fail((msg != null ? msg + ": " : "") + structure(x) + " != " + structure(y));
  else
    print("OK: " + structure(x));
}