type onlyWriter struct {
	io.Writer
}

// A scriptedReader is an io.Reader that executes its steps sequentially.
type scriptedReader []func(p []byte) (n int, err error)

func (sr *scriptedReader) Read(p []byte) (n int, err error) {
	if len(*sr) == 0 {
		panic("too many Read calls on scripted Reader. No steps remain.")
	}