this class is called "TreeShelf" because it is meant to be
used with hierarchical data. Each node in the tree is
specified by a n-ary tuple and the sub-tree rooted in each
node can be retrieved by passing this tuple to __getitem__.
Example: given the list of keys in the tree shelf
[("car","porsche"),("car","dodge"),("animal","bird")]
a call to __getitem__ with argument ("car,") will return a
list of the values associated with the keys
("car","porsche") and ("car","dodge").
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