A Modified Version of 'within'
This is a modified version of the
data.frame method of the generic function
If transformations in 'within' result in an item object, the function returns
a 'data.set' object.
Variables in the
resulting data frame or 'data.set' object will appear in the
order of their creation by the transformation in the call
a data frame.
an expression to evaluate.
further arguments, currently ignored as in the default method.