t.Errorf("shrinking: expected %q got %q", expect, m)
	}

	// 3. Rot13
	m = Map(rot13, []byte("a to zed"))
	expect = "n gb mrq"
	if string(m) != expect {
		t.Errorf("rot13: expected %q got %q", expect, m)
	}

	// 4. Rot13^2
	m = Map(rot13, Map(rot13, []byte("a to zed")))
	expect = "a to zed"
	if string(m) != expect {
		t.Errorf("rot13: expected %q got %q", expect, m)
	}

	// 5. Drop

	w := NewWriterSize(got, 64)
	for i := 0; i < 100; i++ {
		// Obtain a buffer to append to.
		b := w.AvailableBuffer()
		if w.Available() != cap(b) {
			t.Fatalf("Available() = %v, want %v", w.Available(), cap(b))
		}

		// While not recommended, it is valid to append to a shifted buffer.
		// This forces Write to copy the input.
		if rn.Intn(8) == 0 && cap(b) > 0 {
			b = b[1:1:cap(b)]
		}

	// Second, if ContentErr==nil, the content should match Content.
	// If content is large, an alternative to setting Content is to set File,
	// which names a file in the testdata/ directory containing the
	// uncompressed expected content.
	// If content is very large, an alternative to setting Content or File
	// is to set Size, which will then be checked against the header-reported size
	// but will bypass the decompressing of the actual data.

		}
	}
	f.Preamble = strings.Join(linesOut, "\n")
}

// addToFlag appends args to flag.
func (p *Package) addToFlag(flag string, args []string) {
	if flag == "CFLAGS" {
		// We'll also need these when preprocessing for dwarf information.
		// However, discard any -g options: we need to be able
		// to parse the debug info, so stick to what we expect.
		for _, arg := range args {
			if !strings.HasPrefix(arg, "-g") {