reserve a,b,i,k,m,n for Nat;
reserve s,z for non zero Nat;
reserve c for Complex;

theorem Th16:
  k < a implies (a*b+k) mod a = k
  proof
    assume k < a;
    hence k = k mod a by NAT_D:24
    .= (a*b+k) mod a by NAT_D:21;
  end;
