Anti-periodic boundary condition

Hi, I am new to FreeFem and I know one can impose the periodic boundary condition on, say a square, by periodic=[[2, y], [4, y], [1, x], [3, x]] but I am wondering how to impose an “anti-periodic” boundary condition, namely, what if I want to work with functions such that the function restricted on edge 2 (edge 1) is negative of the function restricted on edge 4 (edge 3). More generally, how to impose a boundary condition where (function on edge 2)=(function on edge 4)*g(x,y) where g(x,y) is some other function defined on edge 4?

Thanks a lot!

well, I would say that anti periodicity means that your function is odd by passing through the boundary. So an odd function is zero (Dirichlet) on the boundary…