deffunc H₁( object ) -> set = {$1};
defpred S₁[ object ] means $1 is Ordinal;
let X be set ; :: thesis:
consider Class being set such that
A1: X in Class and
A2: for Y, Z being set st Y in Class & Z c= Y holds
Z in Class and
for Y being set st Y in Class holds
bool Y in Class and
A3: for Y being set holds
( not Y c= Class or Y,Class are_equipotent or Y in Class ) by ZFMISC_1:112;
consider ON being set such that
A4: for x being object holds
( x in ON iff ( x in Class & S₁[x] ) ) from XBOOLE_0:sch 1();
for Y being set st Y in ON holds
( Y is Ordinal & Y c= ON )

let Y be set ; :: thesis:
assume A5: Y in ON ; :: thesis:
hence Y is Ordinal by A4; :: thesis:
reconsider A = Y as Ordinal by A4, A5;
let x be object ; :: according to TARSKI:def 3 :: thesis:
assume A6: x in Y ; :: thesis:
then x in A ;
then reconsider B = x as Ordinal ;
A7: B c= A by A6, ORDINAL1:def 2;
A in Class by A4, A5;
then B in Class by A2, A7;
hence x in ON by A4; :: thesis:

end;

end;

field (RelIncl ON) = ON by Def1;
then consider R being Relation such that
A10: R well_orders Class by A9, Lm1;
consider f being Function such that
A11: ( dom f = X & ( for x being object st x in X holds
f . x = H₁(x) ) ) from FUNCT_1:sch 3();
A12: rng f c= Class

let x be object ; :: according to TARSKI:def 3 :: thesis:
assume x in rng f ; :: thesis:
then consider y being object such that
A13: y in dom f and
A14: x = f . y by FUNCT_1:def 3;
A15: {y} c= X by A11, A13, TARSKI:def 1;
f . y = {y} by A11, A13;
hence x in Class by A1, A2, A14, A15; :: thesis:

end;

let x, y be object ; :: according to FUNCT_1:def 4 :: thesis:
assume that
A17: ( x in dom f & y in dom f ) and
A18: f . x = f . y ; :: thesis:
( f . x = {x} & f . y = {y} ) by A11, A17;
hence x = y by A18, ZFMISC_1:3; :: thesis:

end;

thus ( dom f = X & rng f = rng f ) by A11; :: thesis:

end;