d-test (sample-1 sample-2 tails &key (times 1000) (h0mean 0))
Two-sample test for difference in means. Competes with the unmatched,
two-sample t-test. Each sample should be a sequence of numbers. We calculate
the mean of sample-1' minus the mean of sample-2'; call that D. Under the null
hypothesis, D is zero. There are three possible alternative hypotheses: D is
positive, D is negative, and D is either, and they are selected by the `tails'
parameter, which must be :positive, :negative, or :both, respectively. We count
the number of chance occurrences of D in the desired rejection region, and
return the estimated probability.