Jesica ,

As Ned remarked there are many types of tree data
structures. The Composite pattern chapter in The
Design Patterns Smalltalk Companion book provides some
additional insights. Hierarchical and EToyTextNode
have explicit parent-child structures as does Morph
though it uses subMorph terminology. 

Also, take a look at the FreeDOM class in Morphic
Joules 

http://minnow.cc.gatech.edu/squeak/1968 

It provides a flexible implementation of the Composite
pattern which you can quickly put to use in
representing many of these different types of
hierarchies.  

-Laurence

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