reserve n,k,b for Nat, i for Integer;

theorem Th39:
  7 divides n iff 7 divides
  value(mid(digits(n,10),2,len(digits(n,10))),10) - 2*digits(n,10).0
  proof
    reconsider p=7 as Prime by NAT_4:26;
    10*2 + 1 = p*3;
    hence thesis by Th37,Th22;
  end;
