reserve a,b,c,h for Integer;
reserve k,m,n for Nat;
reserve i,j,z for Integer;
reserve p for Prime;

theorem Th11:
  i,j are_coprime iff -i,-j are_coprime
  proof
    (-i) gcd j = i gcd j & (-i) gcd j = (-i) gcd (-j) by NEWTON02:1;
    hence thesis;
  end;
