
theorem
  for G being _Graph, E being RepDEdgeSelection of G
  holds card E = card Class DEdgeAdjEqRel(G)
proof
  let G be _Graph, E be RepDEdgeSelection of G;
  deffunc F(object) = Class(DEdgeAdjEqRel(G),$1);
  consider f being Function such that
    A1: dom f = E & for x being object st x in E holds f.x = F(x)
    from FUNCT_1:sch 3;
  now
    let y be object;
    hereby
      assume y in rng f;
      then consider x being object such that
        A2: x in dom f & f.x = y by FUNCT_1:def 3;
      y = Class(DEdgeAdjEqRel(G),x) by A1, A2;
      hence y in Class DEdgeAdjEqRel(G) by A1, A2, EQREL_1:def 3;
    end;
    assume y in Class DEdgeAdjEqRel(G);
    then consider x being object such that
      A3: x in the_Edges_of G & y = Class(DEdgeAdjEqRel(G),x) by EQREL_1:def 3;
    set v = (the_Source_of G).x, w = (the_Target_of G).x;
    A4: x DJoins v,w,G by A3, GLIB_000:def 14;
    then consider e being object such that
      A5: e DJoins v,w,G & e in E and
      for e9 being object st e9 DJoins v,w,G & e9 in E holds e9 = e
      by GLIB_009:def 6;
    [x,e] in DEdgeAdjEqRel(G) by A4, A5, GLIB_009:def 4;
    then e in Class(DEdgeAdjEqRel(G),x) by EQREL_1:18;
    then y = Class(DEdgeAdjEqRel(G),e) by A3, EQREL_1:23
      .= f.e by A1, A5;
    hence y in rng f by A1, A5, FUNCT_1:3;
  end;
  then A6: rng f = Class DEdgeAdjEqRel(G) by TARSKI:2;
  now
    let x1,x2 be object;
    assume A7: x1 in dom f & x2 in dom f & f.x1 = f.x2;
    then f.x1=Class(DEdgeAdjEqRel(G),x1) & f.x2=Class(DEdgeAdjEqRel(G),x2)
      by A1;
    then x2 in Class(DEdgeAdjEqRel(G),x1) by A1, A7, EQREL_1:23;
    then [x1,x2] in DEdgeAdjEqRel(G) by EQREL_1:18;
    then consider v,w being object such that
      A8: x1 DJoins v,w,G & x2 DJoins v,w,G by GLIB_009:def 4;
    consider e being object such that
      e DJoins v,w,G & e in E and
      A9: for e9 being object st e9 DJoins v,w,G & e9 in E holds e9 = e
      by A8, GLIB_009:def 6;
    x1 = e & x2 = e by A1, A7, A8, A9;
    hence x1 = x2;
  end;
  hence thesis by A1, A6, CARD_1:70, FUNCT_1:def 4;
end;
