A statistic of one argument, which may be a sample or a distribution.
Dispatching on the argument is what lets mean and variance mean one thing each. A distribution delegates to probability, which owns its own moments; a matrix is treated as a sample. This is why statistics imports probability and not the reverse.
Note the n/n − 1 distinction that follows from the dispatch: variance(sample) is the unbiased estimator, while variance(distribution) is the population variance, because a distribution is not a sample and has no n to correct for. See the package overview.
