term -> expr; solves issue 1676
use factoring on similar terms
use rewriting to see if expr bbecomes a number

This isn't used unconditionally since the user probably doesn't want
the result rewritten in terms of exp. There may be a better way to take
*some* simplification so that even if the expression isn't a number it
will be accepted as a simplification. Like if all the introduced exp
go away then accept the result.