static String commonPrefix(String a, String b) {
  int i = 0, nb = b.length(), na = a.length(), n = min(na, nb);
  while (i < n && a.charAt(i) == b.charAt(i))
    ++i;
  return i == nb ? b : substring(a, 0, i);
}

static S commonPrefix(L<S> l) {
  if (empty(l)) ret "";
  S s = first(l);
  for (int i = 1; i < l(l); i++) {
    if (empty(s)) ret s;
    s = commonPrefix(s, l.get(i)); // Yeah, cold be optimized
  }
  ret s;
}

Began life as a copy of #2000591