return ((TallyStore) tallyi).covariance ((TallyStore) tallyj);
else if (isStable)
return curSum2[i][j - i - 1]/(n-1);
else {
double sum1 = tallyi.sum();
double sum2 = tallyj.sum();
double sum12 = sxy[i][j - i - 1];
return (sum12 - sum1*sum2/n)/(n-1);
}
}