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

theorem
  7 divides n iff 7 divides value(digits(n,10)/^1,10) - 2*digits(n,10).0
  proof
    digits(n,10)/^1=mid(digits(n,10),2,len(digits(n,10))) by Th3;
    hence thesis by Th39;
  end;
