reserve GS for GraphStruct;
reserve G,G1,G2,G3 for _Graph;
reserve e,x,x1,x2,y,y1,y2,E,V,X,Y for set;
reserve n,n1,n2 for Nat;
reserve v,v1,v2 for Vertex of G;

theorem Th4:
  dom (the_Source_of G) = the_Edges_of G & dom (the_Target_of G) =
  the_Edges_of G & rng (the_Source_of G) c= the_Vertices_of G & rng (
  the_Target_of G) c= the_Vertices_of G by FUNCT_2:def 1;
