reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve r,s for Real;
reserve p,p1,p2,p3 for Prime;

theorem
  NatDivisors(p|^4) = {1,p,p|^2,p|^3,p|^4}
  proof
    set A = NatDivisors(p|^4);
    set B = {1,p,p|^2,p|^3,p|^4};
    thus A c= B
    proof
      let x be object;
      assume x in A;
      then ex k st x = k & k <> 0 & k divides p|^4;
      hence thesis by Th85;
    end;
    let m be object;
    assume
A1: m in B;
    then reconsider m as Nat by ENUMSET1:def 3;
    m divides p|^4 by A1,Th85;
    hence thesis by A1;
  end;
