# File lib_core/rucy/document.rb, line 578
    def _umount(mount_map, path, mask=nil)
      unless (mount_map.include? path) then
        raise "not mounted at #{path}."
      end
      node = mount_map[path]

      if (mask) then
        document = nil
        node[:mask_list].delete_if{|m, d|
          if (m == mask) then
            document = d
            true
          end
        }
        unless (document) then
          raise "not mounted at #{path}:#{mask}."
        end
        return document
      else
        unless (node[:document]) then
          raise "not mounted at #{path}."
        end
        document = node[:document]
        node[:document] = nil
        return document
      end
    end