[cairo] Erase a part of a draw

[fab fab]

Hello, I have a question for Cairo-Dock. I'd like to erase a portion of a
surface after I have drawned it. For exemple a function paints a surface on
the context, and then another function wants to make a "hole" in this
surface, totally transparent. How could I achieve that ? I've tried to fill
a path with all the operators, but it never gives me the result I want.
Thanks for your help !
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