reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve r for Real;
reserve p,p1,p2 for Prime;

theorem Th11:
  n divides 9 implies n = 1 or n = 3 or n = 9
  proof
    9 = 3*3;
    hence thesis by Lem2,XPRIMES1:3;
  end;
