static bool startsWith(S a, S b) {
  ret a != null && a.startsWith(b);
}

static bool startsWith(S a, char c) {
  ret nemptyString(a) && a.charAt(0) == c;
}

ifclass Matches
  static bool startsWith(S a, S b, Matches m) {
    if (!startsWith(a, b)) false;
    m.m = new S[] {substring(a, strL(b))};
    true;
  }
endif

static bool startsWith(L a, L b) {
  if (a == null || listL(b) > listL(a)) ret false;
  for (int i = 0; i < listL(b); i++)
    if (neq(a.get(i), b.get(i)))
      ret false;
  ret true;
}