theorem Th23:
  card Edges_Out v = EdgesOut v
proof
  consider X being finite set such that
A1: for z being set holds z in X iff z in the carrier' of G & (the
  Source of G).z = v and
A2: EdgesOut v = card(X) by GRAPH_1:def 22;
  now
    let e be object;
    e in Edges_Out (v, the carrier' of G) iff e in the carrier' of G & (
    the Source of G).e = v by Def2;
    hence e in Edges_Out v iff e in X by A1;
  end;
  hence thesis by A2,TARSKI:2;
end;
