* - ncmul(...,c1,...,c2,...) -> *(c1,c2,ncmul(...)) (pull out commutative elements)
* - ncmul(x1,y1,x2,y2) -> *(ncmul(x1,x2),ncmul(y1,y2)) (collect elements of same type)
* - ncmul(x1,x2,x3,...) -> x::eval_ncmul(x1,x2,x3,...)
* - ncmul(...,c1,...,c2,...) -> *(c1,c2,ncmul(...)) (pull out commutative elements)
* - ncmul(x1,y1,x2,y2) -> *(ncmul(x1,x2),ncmul(y1,y2)) (collect elements of same type)
* - ncmul(x1,x2,x3,...) -> x::eval_ncmul(x1,x2,x3,...)