[vtkusers] Execute/ExecuteInformation and Set*Extent with vtkRectilinearGrids

[tom fogal]

I'm wondering what the difference is between Execute
and ExecuteInformation, particularly in a source object.
They seem to be functions that are to be called when a change
in the pipeline has occurred, but I can't seem to grasp the
correct behavior for them. Should I just call Execute on all
of my object's outputs?

I was also wondering what SetExtent and SetWholeExtent tell
an object about the data. Is it the number of elements in 
the input arrays, or perhaps the range of possible values in
those input arrays?
RGrid.cxx from the examples seems to imply that it tells the
object about the number of values in the array, but maybe
this is just for a vtkRectilinearGridGeometryFilter and the
function takes on a different meaning for a vtkRectilinearGrid
(the type of source I'm trying to make)?

Any further information on either of those two concepts would
be greatly appreciated. Thanks,

-tom