const endOfText rune = -1

// input abstracts different representations of the input text. It provides
// one-character lookahead.
type input interface {
	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.