add subs rule and tool
remove SymPy dependence from traverse

All dependence is now in util.py
add exhaustive branching strategy
add branching debug strategy
add multiplex branching strategy
add condition branching strategy
add notempty branching strategy
add traversal branching strategy
rename branching -> branch
add canon branching strategy
move branch files to new directory
AST -> tree
file is None -> not File and debug docstring
move ident_is_even into test function
"rule" -> "branching rule"
Brief header comments for rules files
remove excess newline from end of file
product import itertools -> sympy.compatibility
sympy.compatibility -> sympy.core.compatibility
add rules.branch to setup.py