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AutoRecalcDict
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An AutoRecalcDict is a subclass of dict that allows the user to create a
standard python dict, but to also allow specifications for dependency
lists that are on specific targets. Typical usage would be when
you want a dict entry to automagically be recomputed if a dependency
changes.

For example:
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ard is an AutoRecalcDict and has keys called 'target', 'dep1', 'dep2' and
'dep3'. The user wants the value of ard['target'] to be recomputed via
a user defined function ff.

The calling sequence of function ff is defined as having three
arguments:
1. self (ff will be converted to an instance method of ard)
2. target (the index into ard)
3. deplist (the list of keys that target depends upon).

Included in autorecalcdict.py is a class called DepDescFunc. This is 
used to create the instance needed by AutoRecalcDict to know what
targets are dependent on which dependency lists and the function
needed to update the target.

See the module for examples of usage.
