code: protected <T extends InvocationMarshaller<?>> T addProvider (
          InvocationProvider prov, Class<T> mclass)
      {
          T marsh = _invmgr.registerProvider(prov, mclass);
          _marshallers.add(marsh);
          return marsh;
      }
docs: Registers an invocation provider and notes the registration such that it will be
  automatically cleared when this manager shuts down.