def lower
      position? ?
          siblings.where(klass.arel_table[klass.ordered_tree.columns.position].gteq(position)) :
          siblings
    end
def lower
      position? ?
          siblings.where(klass.arel_table[klass.ordered_tree.columns.position].gteq(position)) :
          siblings
    end
Returns all nodes that are lower than current position