fitted model object of class
"tree".
This is assumed to be the result of some function that produces
an object with the same named
components as that returned by the
tree function.
VALUE:
Returns the data that was used to grow the supplied tree. If
the supplied tree is not a direct result of the function
tree,
then the function recursively looks back to find the original call
and returns the corresponding data frame.
This function is not intended
to be called directly by users.