reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;

theorem Th2:
  Seg n c= Segm(n+1)
proof
  let x be object;
  assume
A1: x in Seg n;
  then reconsider x as Element of NAT;
  x<=n by A1,FINSEQ_1:1;
  then x<n+1 by NAT_1:13;
  hence thesis by NAT_1:44;
end;
