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 Th13:
  n divides 25 implies n = 1 or n = 5 or n = 25
  proof
    25 = 5*5;
    hence thesis by Lem2,XPRIMES1:5;
  end;
