simplify d(a^2*y^2)/d(a*y) with a constant

Dear list-users,

I would like to know if it is possible to use Maxima to non-dimensionalize a
differential equation.

Let say I have the expression

expr:x+d(x^2)/dx

I want to replace x by y with y=a*x

I have used the following to do that

subst(a*y,x,expr);

The answer is

a*y+d(a^2*y^2)/d(a*y)

Is it possible to simplify the last term since a is constant?

Thank you

C?dric