def next_root_lft
      last_root = nested_interval_scope.roots.order( rgtp: :desc, rgtq: :desc ).first
      raise Exception.new("Only one root allowed") if last_root.present? && !self.class.nested_interval.multiple_roots?
      last_root.try(:right) || 0.to_r
    end
def next_root_lft
      last_root = nested_interval_scope.roots.order( rgtp: :desc, rgtq: :desc ).first
      raise Exception.new("Only one root allowed") if last_root.present? && !self.class.nested_interval.multiple_roots?
      last_root.try(:right) || 0.to_r
    end
Returns left end of interval for next root.