reify
__repr__ for unify.core classes
rewrites first commit
add test for strategy
top_down parametrizes children and op
tree traversal functions for unify/core
add statsimp
factor out common Density call in rewriterules
density -> distribution in simplify
add unpack_Density rule
Use rebuild rule.  Simplify tests
Testing statsimp

chisquared example works
testing expression simplification separately from densities
changed strategies in statsimp
factor out additive versions of rules
statsimp uses top_down and factor
wilds -> variables in statsimp tests
factor multiplex to interleave, unique, fnmap
listcomp to map zip
more patterns for normal rvs
statsimp uses new rewriterules

10x performance increase