Substitution question



I have the following result after some calculations:

myvar:(Gact*Gth*(Ga*s^2*xbeam+thdhat))/(Gact*Gth+1);

I would like to define a new variable Gcl and substitute it for the
expression Gact*Gth/(Gact*Gth+1) in the above result.  I don't seem to
be able to do this easily with any substitution commands.  Is there an
easy way to do this substitution?  The only way I have found is to do
the following:

Gcl2:Gact*Gth/(Gact*Gth+1);

myvar2:myvar/Gcl2*Gcl;

which produces the desired result:
Gcl*(Ga*s^2*xbeam+thdhat)

but this would be a very bad approach if it were accidentally applied
to an expression that didn't already contain the term I am trying to
factor out.

Any thoughts?

Thanks,

Ryan