How to avoid this gradef



Hello,

I was wondering if there is a more elegant way than the following:


depends([r,t],[x,y])$ depends(phi,[r,t])$

(gradef(r,x,cos(t)),    gradef(r,y,sin(t)),
 gradef(t,x,-sin(t)/r), gradef(t,y,cos(t)/r))$

trigreduce(diff(phi,x,2)+diff(phi,y,2));


in order to calculate the laplacian in polar coordinates. I do not want to use 
that gradef() for specifying the derivatives of r and t, given that they should 
be defined something like

r(x,y) := sqrt(x^2+y^2); t(x,y) := atan2(y,x);

It would be nice if I can have the last line in my code

Thank you very much for your help,

Diego