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.