Simplifying/gathering expression on known variables

I am trying to do some analytical math with matlab using the symbolic toolbox, and at the end gather/simplifying the result using the known variables and expressions I have.

As an example to show what I want to do (this is just an example, not my actual code):

syms x1 x2 a b c x3 x2 = x3+cos(x1)*b; t = a*b*x1 + 30 + b + c*x2; %t will now be: b + c*(x3 + b*cos(x1)) + a*b*x1 + 30

My question is: How can I simplify this expression again to get back the inital expression a*b*x1 + 30 + b + c*x2 ? In other words, how can I make matlab replace x3+cos(x1)*b by x2 again? I have tried the simplify and collect functions, but they do not replace x3+cos(x1)*b by x2.

-Ivar