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

theorem Th9:
  -1 mod 3 = 2
  proof
    -1 - ((-1) div 3) * 3 = 2 by Th8;
    hence thesis by INT_1:def 10;
  end;
