Simplified Graph trait

Graph.scala

trait Graph[V, E] {
  type Col[A]
  type Incidence[A]
  
  def vertices: Col[V]
  def edges: Col[E]
  def incidenceOption(e: E): Incidence[V]
}

GraphT.scala

trait GraphT[G, V, E] {
  type Col[A]
  type Incidence[A]
  
  def vertices(g: G): Col[V]
  def edges(g: G): Col[E]
  def incidenceOption(g: G, e: E): Incidence[V]
}