Context.Rem fails for Rem(100E100, 10)

[NoamTD]

I attempted to run the following test:

func TestRem(t *testing.T) {
	a := apd.New(100, 100)
	b := apd.New(10, 1)
	var rem apd.Decimal
	_, err := apd.BaseContext.Rem(&rem, a, b)
	if err != nil {
		t.Fatal(err)
	}

	if !rem.IsZero() {
		t.Fatal("expected zero")
	}
}

I would expect this test to pass, but it fails:

=== RUN   TestRem
    rem_test.go:72: division impossible
--- FAIL: TestRem (0.00s)

FAIL

apd.BaseContext.Rem returns division impossible, which (I believe) is an incorrect result in this case.

[mvdan]

For some added context, we wanted to use Rem to be able to precisely tell if one apd.Decimal number was a multiple of another. We instead ended up with the following code, requiring two apd operations (Quo + Modf) instead of one (Rem):

_, err := apdCtx.Quo(&d, x, y)
if err != nil {
	return false, err
}
var frac apd.Decimal
d.Modf(nil, &frac)
return frac.IsZero(), nil