A zipper is a data structure that lets you efficiently "modify" an
immutable data structure. "Modify", because a zipper creates a new,
updated, data structure that reuses as much of the original structure.
Local modifications and navigations are constant time.

This zipper implementation lets you walk over an arbitrary graph
structure by leaving the decision of what "up", "down", "left" and
"right" means to you, the programmer.

(Zippers should also work just fine in Squeak 4.3, Pharo 1.4 and Pharo 2.0.)

frank