fix wester test
apart and cancel are nc-aware
2699: use xthreaded, not threaded in apart
denest powers in powsimp so 4**s*2**(x-2*s) -> 2**x

Should this only be done if the base is a Number?
3341: allow factors of coeff to enter powers

* i order to make this work, multiplicity was made to handle Rationals.