How can we express union of two or more constraints, for example $x\ge 0 \vee y\ge 0$, in an inequality system as below:
$f_1(x,y)\ge 0 \\f_2(x,y)\ge 0 \\f_3(x,y)\ge 0 \\ \qquad\vdots$
where $f_i\in \mathbb{C}^1$. This feasible set is not necessarily a convex set, hence there is not any linear inequality system for this problem. Is there any non-linear one?