 However, because the exact set of forbidden minors for this property is not known, this proof of existence is non-constructive, and does not lead to an explicit description of the set of forbidden minors or of the algorithm based on them.Another graph operation that preserves the existence of a planar cover is the Y-Δ transform, which replaces any degree-three vertex of a graph "H" by a triangle connecting its three neighbors.However, the reverse of this transformation, a Δ-Y transform, does not necessarily preserve planar covers.