# Using implicit_plot with non-rectangular regions

I'd like to use implicit_plot on a function that involves a square root. When I do, I get an error message saying that for some values of x and y the number inside the square root is negative. Well, sure. Is there a way to restrict implicit_plot to the region where the function has real values? In this the thing in the square root is 1-u^2-v^2, so I'd like to restrict u and v to the unit circle.

Could you post the exact code you are using? I've tried some examples with functions involving such a square root and SageMath skips regions where the root is not defined without yielding errors.