## "easy" problem

I'm quite embrassed for asking this, but these year we had two extra lessons at the uni about matlab... my prof said, that we don't need to participate, but now we have a homework, that we're required to do... T.T
so i have this:

function elli(fun,a,n)
%
if n<4
disp('Legyen n>3')
return
end
if a<= 0
disp('legyen a>0')
return
end

x=linspace(-a,a,n);
[xx,yy]=meshgrid(x);
A=feval(fun,xx,yy);
B=zeros(n,n);

aa=a*a;
for i=1:n
xixxi=x(i)*x(i);
for j=1:n
if xixxi+x(j)*x(j)>aa
A(i,j)=NaN; B(i,j)=NaN;
end
end
end

mesh(B);
hold on
mesh(A)
hold off

and i would have to explain, what this "elli" does...
I would be really greatful for your help...