Substitution of dependend variable in differentiation



Hi everybody,

I have got a function u depending on x which I differentiate with 
respect to x.  I then want to substitute  u(x)=l*U(x),  in order to get  
du/dx = l*dU/dx.  What I have done so far is :

(%1)       expr1:diff (u(x),x)
(%2)       expr2:subst(l*U(x),u(x),expr1)
(%o2)     d(l*U(x)) / dx

How can I obtain my desired result?

Many thanks in advance.

Marc Hodapp