I have an expression 

f:= x1 * x2 * x3^4;

I would like to do something like subs( (x1,x3) = (x3,x1), f) and get 

x1^4 * x2 * x3

back.

However this does not work. Is there some _lightweight_ way of doing it?

I have a set E of linear equations in variables x[1],...,x[8] (which I created with X := Matrix(8,1,symbol=x)). I now call

> solve(E);

I get back something like

x[1] = x[2]-x[3], x[2] = x[2], x[3] = 5, ...

I would rather like to get

x[1] = t[1] - 5, x[2] = t[1], x[3] = 5, ...

back.

How to do this? I do want to use the solve function (not LinearSolve et.c.).