assertTrue(Strings.validSurrogatePairAt("\uD8AB\uDCAB", 0));
    assertTrue(Strings.validSurrogatePairAt("abc\uD8AB\uDCAB", 3));
    assertTrue(Strings.validSurrogatePairAt("abc\uD8AB\uDCABxyz", 3));
    assertFalse(Strings.validSurrogatePairAt("\uD8AB\uD8AB", 0));
    assertFalse(Strings.validSurrogatePairAt("\uDCAB\uDCAB", 0));
    assertFalse(Strings.validSurrogatePairAt("\uD8AB\uDCAB", -1));

    checkNotNull(b);

    int maxPrefixLength = min(a.length(), b.length());
    int p = 0;
    while (p < maxPrefixLength && a.charAt(p) == b.charAt(p)) {
      p++;
    }
    if (validSurrogatePairAt(a, p - 1) || validSurrogatePairAt(b, p - 1)) {
      p--;
    }
    return a.subSequence(0, p).toString();
  }

  /**
   * Returns the longest string {@code suffix} such that {@code a.toString().endsWith(suffix) &&