static int indentWidth(int spacesPerTab default 2, S s) {
  int count = 0, n = l(s);
  for i to n: {
    char c = s.charAt(i);
    if (c == ' ') ++count;
    else if (c == '\t') count += spacesPerTab;
    else break;
  }
  ret count;
}

Began life as a copy of #1020372