How to define a Function in Maxima



Hi all,

I'm very new to Maxima, and hence quite lost in so many function 
declarations. I want to define a function as the following and want to 
plot it:

f(x,y) = x*y/(x^2+y), if (y != -x^2) and 0 otherwise

How to put it in Maxima? Thanks in advance ...

Swaprava