The permutation schema is defined using the `how` function. The implementation of this function is inspired from the VEGAN package and reproduced here to avoid an extra dependency on an hidden vegan function.
.getPermuteMatrix(permutations, n, strata = NULL)a list of control values for the permutations as returned
by the function how, or the number of
permutations required.
numeric; the number of observations in the sample set.
May also be any object that nobs knows about;
see nobs methods.
A factor, or an object that can be coerced to a factor via as.factor, specifying the strata for permutation.