set X = Seg n;
Seg n c= NAT

proof

let x be object ; :: according to TARSKI:def 3 :: thesis: ( not x in Seg n or x in NAT )
assume x in Seg n ; :: thesis: x in NAT
then ex k being Nat st
( x = k & 1 <= k & k <= n ) ;
hence x in NAT by ORDINAL1:def 12; :: thesis: verum

end;

hence Seg n is Subset of NAT ; :: thesis: verum