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 Th21:
  Edges_Out(v, X) c= Edges_Out v
proof
  let e be object;
  assume
A1: e in Edges_Out(v, X);
  then (the Source of G).e = v by Def2;
  hence thesis by A1,Def2;
end;
