// of length j at offset i and ask for
				// Next(k), we get the right bytes.
				buf := NewBuffer(b[0:j])
				n, _ := buf.Read(tmp[0:i])
				if n != i {
					t.Fatalf("Read %d returned %d", i, n)
				}
				bb := buf.Next(k)
				want := k
				if want > j-i {
					want = j - i
				}
				if len(bb) != want {
					t.Fatalf("in %d,%d: len(Next(%d)) == %d", i, j, k, len(bb))
				}