the standard defines corner cases when it should return null - but the current result is NaN.
If N * SUMX2 equals SUMX * SUMX , then the result is the null value.
If N * SUMY2 equals SUMY * SUMY , then the result is the null value.
returns 0 instead 1
`If N is 1 (one), then the result is the null value.`
the standard uses (y,x) order; and some of the function names are also contain X and Y...so the order does matter..currently at least corr uses (x,y) order which is okay - because its symmetric; but it would be great to have the same order everywhere (check others)