theorem
  Index (1).G = card G
proof
  deffunc F(object) = {$1};
  consider f being Function such that
A1: dom f = the carrier of G and
A2: for x being object st x in the carrier of G holds f.x = F(x)
    from FUNCT_1:sch 3;
A3: rng f = Left_Cosets (1).G
  proof
    thus rng f c= Left_Cosets (1).G
    proof
      let x be object;
      assume x in rng f;
      then consider y being object such that
A4:   y in dom f and
A5:   f.y = x by FUNCT_1:def 3;
      reconsider y as Element of G by A1,A4;
      x = {y} by A2,A5;
      then x in the set of all {a};
      hence thesis by Th138;
    end;
    let x be object;
    assume x in Left_Cosets (1).G;
    then x in the set of all {a} by Th138;
    then consider a such that
A6: x = {a};
    f.a = {a} by A2;
    hence thesis by A1,A6,FUNCT_1:def 3;
  end;
  f is one-to-one
  proof
    let x,y be object;
    assume that
A7: x in dom f & y in dom f and
A8: f.x = f.y;
    f.y = {y} & f.x = {x} by A1,A2,A7;
    hence thesis by A8,ZFMISC_1:3;
  end;
  then the carrier of G,Left_Cosets (1).G are_equipotent
    by A1,A3,WELLORD2:def 4;
  hence thesis by CARD_1:5;
end;
