sS replace_multi(S s, Cl<S> a, S b) {
  if (s == null || empty(a) || b == null) ret s;
  for (S aa : a)
    s = s.replace(aa, b);
  ret s;
}

// replacements = in, out, in, out, ...
sS replace_multi(S s, S... replacements) {
  if (empty(s) || empty(replacements)) ret s;
  for (int i = 0; i+1 < l(replacements); i += 2)
    s = s.replace(replacements[i], replacements[i+1]);
  ret s;
}

Began life as a copy of #1007775