BUG: (fixup) There is no 'operator.mull'
ENH: Refactor frame flex methods to common func
Rename arith_factory --> arith_method, shorten docstring
have add_special_arithmetic_methods add comp_methods too
FIXUP: change other part of arith_factory --> arith_method
Only add if method not already in class' __dict__
Rename flex_factory --> flex_method in common
Add flex method - only add if name not in class' __dict__
Frame: refactor rich comparison to common
Series: refactor rich comparisons to common