func (con *Reference) Visit(visitor Visitor) error {
	return visitor.VisitReference(con)
}
Visit calls VisitReference.
package booklit
import (
	"fmt"
	"github.com/vito/booklit/ast"
)
type Reference struct {
	TagName string
	Content Content
	Tag *Tag
	Optional bool
	Location ast.Location
}
func (con *Reference) IsFlow() bool {
	return true
}
func (con *Reference) String() string {
	if con.Content != nil {
		return con.Content.String()
	}
	if con.Tag != nil {
		return con.Tag.Title.String()
	}
	return fmt.Sprintf("{reference: %s}", con.TagName)
}