}
	t.inst = i
	return t
}

// A lazyFlag is a lazily-evaluated syntax.EmptyOp,
// for checking zero-width flags like ^ $ \A \z \B \b.
// It records the pair of relevant runes and does not
// determine the implied flags until absolutely necessary
// (most of the time, that means never).
type lazyFlag uint64

func newLazyFlag(r1, r2 rune) lazyFlag {
	return lazyFlag(uint64(r1)<<32 | uint64(uint32(r2)))
}

	step(pos int) (r rune, width int) // advance one rune
	canCheckPrefix() bool             // can we look ahead without losing info?
	hasPrefix(re *Regexp) bool
	index(re *Regexp, pos int) int
	context(pos int) lazyFlag
}

// inputString scans a string.
type inputString struct {
	str string
}

func (i *inputString) step(pos int) (rune, int) {
	if pos < len(i.str) {
		c := i.str[pos]