fact: Definition lr_door (c1 c2 : cell) : vert_edge := head dummy_vert_edge (filter (fun x => existsb (fun x' => vert_edge_eqb x x') (cell_safe_exits_left c2)) (cell_safe_exits_right c1)).
imports: mathcomp(all_ssreflect) ZArith QArith List String OrderedType OrderedTypeEx FMapAVL