An object of class "ssf" represents a
function (real- or vector-valued) that has been
sampled at a finite set of points.
It contains a data frame
which provides the function values
at the sample points.
In with.ssf, the expression specified by …
will be evaluated in this dataframe.
In apply.ssf, the dataframe will be subjected to
the apply operator using the additional arguments
….
If the result of evaluation
is a data frame with one row for each data point,
or a numeric vector with one entry for each data point,
then the result will be an object of class "ssf"
containing this information. Otherwise, the result will be
a numeric vector.