// An EmptyOp specifies a kind or mixture of zero-width assertions.
type EmptyOp uint8

const (
	EmptyBeginLine EmptyOp = 1 << iota
	EmptyEndLine
	EmptyBeginText
	EmptyEndText
	EmptyWordBoundary
	EmptyNoWordBoundary
)

// EmptyOpContext returns the zero-width assertions
// satisfied at the position between the runes r1 and r2.
// Passing r1 == -1 indicates that the position is
// at the beginning of the text.

	case OpBeginText:
		return c.empty(EmptyBeginText)
	case OpEndText:
		return c.empty(EmptyEndText)
	case OpWordBoundary:
		return c.empty(EmptyWordBoundary)
	case OpNoWordBoundary:
		return c.empty(EmptyNoWordBoundary)
	case OpCapture:
		bra := c.cap(uint32(re.Cap << 1))
		sub := c.compile(re.Sub[0])
		ket := c.cap(uint32(re.Cap<<1 | 1))
		return c.cat(c.cat(bra, sub), ket)
	case OpStar: