// 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();
  if (m == 0 || n == 0) ret Double.NaN;
  double sum = 0;
  for (A key : joinSets(keys(ms1), keys(ms2))) {
    double mi = ms1.get(key);
    double ni = ms2.get(key);
    double item = sqr(m*ni-n*mi)/(mi+ni);
    ifdef chiSquared_debug
      printVars_str(+key, +mi, +ni, +item);
    endifdef
    sum += item;
  }
  ifdef chiSquared_debug
    printVars_str(+sum, mn := m*n, mPlusN := m+n);
  endifdef
  ret sum/(m*n);
  //ret sum/(m+n);
  //ret sum/pow(m*n, 1.5);
}