def [](intfname)
      introspect unless introspected
      ifc = @interfaces[intfname]
      raise DBus::Error, "no such interface `#{intfname}' on object `#{@path}'" unless ifc
      ifc
    end
def [](intfname)
      introspect unless introspected
      ifc = @interfaces[intfname]
      raise DBus::Error, "no such interface `#{intfname}' on object `#{@path}'" unless ifc
      ifc
    end
Retrieves an interface of the proxy object
 @param [String] intfname
 @return [ProxyObjectInterface]