code: func attributeDashmatchSelector(key, val string) Selector {
  	return attributeSelector(key,
  		func(s string) bool {
  			if s == val {
  				return true
  			}
  			if len(s) <= len(val) {
  				return false
  			}
  			if s[:len(val)] == val && s[len(val)] == '-' {
  				return true
  			}
  			return false
  		})
  }
docs: // attributeDashmatchSelector returns a Selector that matches elements where
  // the attribute named key equals val or starts with val plus a hyphen.