Use a dictionary to implement switch/case sematics 
instead of a long if/elif chain.

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Gustavo, what do you think?

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Raymond, have you done any performance tests showing that the 
proposed scheme is better? 
 
I've created a small test bed for the two patterns here and it looks like the 
current pattern is faster for small sets. Given the fact that this is a 
recursive pattern, and that I'm unsure about the kind of patterns which 
are most used, I'm a little bit uncomfortable in applying this. 
 

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It needs to be timed on very large patterns -- it will
always do worse on very small patterns (due to the time to
setup the dispatch table).

I'm not satisfied with the patch the way it stands. 
Ideally, the dispatch table needs to be pre-computed at
compile time.  The trick is how to view the enclosing
variables (passing them as arguments is slow; using globals
is not especially clean or fast).  If that were resolved,
then this approach would just about always be faster (a
dictioary lookup and function call versus a long chain of
elifs).

Any suggestions are welcome.

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I can't see any way to fix the long if/elif chain without 
introducing switch/case.  The dispatch idiom is too expensive 
in terms of function call overhead and data sharing costs.