how to solve for an implicit derivative --

Hello maxima users -- 

This should be easy but in a couple hours of messing around I haven't 
figured it out - 

f(x,y)=0;

depends(y,x);

eq1: diff(f,x);

(gves an expression involving dy/dx)

How can I 

solve(eq1, dy/dx);

I'm asking here about syntax -- I understand that for many choices of f, 
solve will fail. 
For discussion, take f(x,y) = x^2 + y^2 -1.

Thanks, 

Steve