 reserve n,i,k,m for Nat;
 reserve p for Prime;

theorem
  not n+1 in SetPrimes n
  proof
    assume n+1 in SetPrimes n; then
    n+1 in Seg n by XBOOLE_0:def 4; then
    n+1 <= n by FINSEQ_1:1;
    hence thesis by NAT_1:13;
  end;
