# File lib_core/rucy/document.rb, line 680
    def virtual_attach(host, filter, path, mask=nil)
      host += ':80' if (host !~ /:\d+$/)
      unless (@virtual_mount_map.include? host) then
        raise "not mounted virtual host: #{host}"
      end
      _attach(@virtual_mount_map[host], filter, path, mask)
    end