static SS matchDollarVars(S pat, S s) {
  new L<S> vars;
  S pat2 = dollarVarsToStars(pat, vars);
  new Matches m;
  if (!match(pat2, s, m)) null;
  new SS map;
  for (int i = 0; i < l(m.m); i++) {
    S var = vars.get(i);
    S value = m.get(i);
    S oldValue = map.get(var);
    if (oldValue == null) map.put(var, value);
    else if (neq(oldValue, value)) null;
  }
  ret map;
}

Began life as a copy of #1010855