func (rs *RuneSet) Contains(r rune) bool {
	return rs.set[r]
}
Contains returns true iff r is in rs
package runeset
import (
	"bytes"
	"fmt"
	"sort"
)
type RuneSet struct {
	set map[rune]bool
}
func New() *RuneSet {
	return &RuneSet{
		set: make(map[rune]bool),
	}
}
func (rs *RuneSet) Add(r rune) {
	rs.set[r] = true
}