[Maxima] complex expressions

[Henry Baker]

I'm trying to get maxima to prove that

abs(a)^2=realpart(a)^2+imagpart(a)^2.

I tell maxima:

declare(a,complex);
abs(a)^2=realpart(a)^2+imagpart(a)^2;

What do I tell maxima to get it to simplify this?