constructor
V : Type u_1
G G' : Digraph V
⊢ G < G' ↔ ∃ s, Set.Nonempty s ∧ s ⊆ edgeSet G' ∧ G = deleteEdges G' s
case mp
V : Type u_1
G G' : Digraph V
⊢ G < G' → ∃ s, Set.Nonempty s ∧ s ⊆ edgeSet G' ∧ G = deleteEdges G' s

case mpr
V : Type u_1
G G' : Digraph V
⊢ (∃ s, Set.Nonempty s ∧ s ⊆ edgeSet G' ∧ G = deleteEdges G' s) → G < G'