How to exploit symmetry in FEM problem (using assempde)?

I need to solve a basic elliptic boundary value problem using finite elements. assempde works just fine but since I have to perform a lot of calculations, I would like to exploit the symmetry of my problem at hand (it is symmetric about the y-axis) to reduce the computational effort.

Is there a clever way to do so (using assempde)?

Thank you!

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Nobody as any idea?