eulerCharOf :: (SurfaceDiagram a) => a t -> Int
eulerCharOf a = numberOfVertices a + numberOfFaces a - numberOfEdges a
eulerCharOf :: (SurfaceDiagram a) => a t -> Int
eulerCharOf a = numberOfVertices a + numberOfFaces a - numberOfEdges a
eulerCharOf a = numberOfVertices a + numberOfFaces a - numberOfEdges a