static S centerPad(O s, int l) {
  ret centerPad(s, l, ' ');
}

static S centerPad(O s, int l, char c) {
  S _s = str(s);
  if (l(_s) >= l) ret _s;
  int total = l-l(_s), left = total/2;
  ret rep(c, left) + _s + rep(c, total-left);
}