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.
modifications to simplify

== add quick exit for simplify if expr is Atom

== 3712: try return factored expression more often

== 3730: use as_content_primitive and cancel
upgrade solving for abs, powers, and real/imag variables

Also:

    use powdenest before factoring in _tsolve

    fix detection of difference of functions