// Formula: 1/(MN) SUM_i[((Mni - Nmi)^2)/(mi+ni)]
static <A> double chiSquared(MultiSet<A> ms1, MultiSet<A> ms2) {
  int m = ms1.size(), n = ms2.size();
  double sum = 0;
  for (A key : joinSets(keys(ms1), keys(ms2))) {
    double mi = ms1.get(key);
    double ni = ms2.get(key);
    double item = sqr(mi-ni)/(mi+ni);
    ifdef chiSquared_debug
      printVars_str(+key, +mi, +ni, +item);
    endifdef
    sum += item;
  }
  ret 1/(sum*m*n);
}