func TestFailing(t *testing.T) {
	type s struct {
		i   int
		err error
	}
	tcs := []s{
		{1, m3.ErrorLT3},
		{2, m3.ErrorLT3},
		{-1, m3.ErrorLT3},
		{-10, m3.ErrorLT3},
		{2002, m3.ErrorEven},
		{-2002, m3.ErrorEven},
		{0, m3.ErrorLT3},
	}
	for _, tc := range tcs {
		m, err := m3.Digits(tc.i)
		if err == nil {
			t.Fatal("d(%d) expected error %q, got non-error %q.",
				tc.i, tc.err, m)
		}
		if err != tc.err {
			t.Fatal("d(d) expected error %q, got %q", tc.i, tc.err, err)
		}
		t.Logf("d(%d) returns %q", tc.i, err)
	}
}
func TestPassing(t *testing.T) {
	type s struct {
		i int
		m string
	}
	tcs := []s{
		{123, "123"},
		{12345, "234"},
		{1234567, "345"},
		{987654321, "654"},
		{10001, "000"},
		{-10001, "000"},
	}
	for _, tc := range tcs {
		m, err := m3.Digits(tc.i)
		if err != nil {
			t.Fatalf("d(%d) returned %q.", tc.i, err)
		}
		if m != tc.m {
			t.Fatalf("d(%d) expected %q, got %q.", tc.i, tc.m, m)
		}
		t.Logf("d(%d) = %q.", tc.i, m)
	}
}