reserve G for Graph,
  v, v1, v2 for Vertex of G,
  c for Chain of G,
  p, p1, p2 for Path of G,
  vs, vs1, vs2 for FinSequence of the carrier of G,
  e, X for set,
  n, m for Nat;

theorem Th17:
  e in the carrier' of G & e in X implies G-VSet X is non empty
proof
  assume that
A1: e in the carrier' of G and
A2: e in X;
  reconsider v = (the Source of G).e as Vertex of G by A1,FUNCT_2:5;
  v in G-VSet X by A1,A2;
  hence thesis;
end;
