reserve k for Nat;
reserve p for Prime;

theorem
  p*p <= k < 49 implies p = 2 or p = 3 or p = 5
  proof
    assume p*p <= k < 49;
    then p*p < 7*7 by XXREAL_0:2;
    hence thesis by Ttool7a,NAT_4:1;
  end;
