Sorry, Approach 3 is really Approach 2A -- the argument to substpart
shouldn't be expr, but the result of Approach 2:
> ----------------Approach 2-------------------------------
> expr1: subst(x-a,'xa,ratsubst('xa,x-a,expr)); <<< ratsubst does the
> heavy lifting
> => (3*(x-a)^2-6)*sin(x)+(6*(x-a)-(x-a)^3)*cos(x)
>
> -----------------Approach 2A------------------------------
> pullout(ex,q):=q*multthru(1/q,ex)$
> substpart(pullout(piece,x-a),expr1,2,1); <<< pull out x-a from the 2nd
> term
> => (3*(x-a)^2-6)*sin(x)+(x-a)*(6-(x-a)^2)*cos(x)
>
>