reserve x, y, z, v for set,
  n, m, k for Nat;
reserve G, G1, G2, G3 for Graph;
reserve x, y for Element of (the carrier of G);

theorem
  for G being Graph holds dom(the Source of G) = the carrier' of G &
  dom(the Target of G) = the carrier' of G by FUNCT_2:def 1;
