r := &Rat{Int{abs: nat{991}}, Int{abs: make(nat, 0, 1)}}
		acc.f(r)
		if d := r.b.abs[:1][0]; d != 0 {
			t.Errorf("%s modified denominator: got %d, want 0", acc.name, d)
		}
	}
}

func TestDenomRace(t *testing.T) {
	x := NewRat(1, 2)
	const N = 3
	c := make(chan bool, N)
	for i := 0; i < N; i++ {
		go func() {
			// Denom (also used by Float.SetRat) used to mutate x unnecessarily,