OO and large packages

[tmoran@bix.com (Tom Moran)]

Is there a good methodology to break up a complex object into multiple
smaller packages?
  In a windowing system I have a "window" object with a great many
functions for both input and output, some of which are rather complex.
One possible split is 
  type output_window is new basic_window ...
and a package providing just the output functions, then another
package with
  type input_window is new output_window ...
overiding the input functions of basic window (which weren't even
mentioned in input_window).  That produces two, smaller, package, but
output_window is secretly passing all the input functions from
basic_window on to input_window, which seems a really poor idea.
  The only other obvious breakup is to include everything in one large

  type io_window is new basic_window ...
but move the guts of the larger functions off to separate packages in
a traditional bottom-up way, with no special conceptual structure.
  Suggestions?