static S collapseWord(S s) {
  if (s == null) ret "";
  new StringBuilder buf;
  for (int i = 0; i < l(s); i++)
    if (i == 0 || neq(substring(substring(s, i, i+1), substring(s, i-1, i))))
      buf.append(s.charAt(i));
  ret buf.toString();
}