ed to compute the statistics.  If this is empty,
a default set of summary statistics is used.  Functions used must accept
a vector of values and return either a (possibly named) single value,
a (possibly named) vector of values, or a data frame with one row.
Functions can be specified with character strings, names, or expressions
that look like function calls with the first argument missing.  The latter
option provides a convenient way to specify additional arguments.  See the
examples.
Note: If these argum