use G'.edgeSet \ G.edgeSet
case mp
V : Type u_1
G G' : Digraph V
h : G < G'
⊢ ∃ s, Set.Nonempty s ∧ s ⊆ edgeSet G' ∧ G = deleteEdges G' s
case mp
V : Type u_1
G G' : Digraph V
h : G < G'
⊢ Set.Nonempty (edgeSet G' \ edgeSet G) ∧
    edgeSet G' \ edgeSet G ⊆ edgeSet G' ∧ G = deleteEdges G' (edgeSet G' \ edgeSet G)