code: private <Q extends BaseProxy> SimpleProxyId<Q> allocateSyntheticId(final String typeToken,
        final int syntheticId) {
      @SuppressWarnings("unchecked")
      SimpleProxyId<Q> toReturn = (SimpleProxyId<Q>) this.state.syntheticIds.get(syntheticId);
      if (toReturn == null) {
        toReturn = this.state.requestFactory
            .allocateId(this.state.requestFactory.<Q>getTypeFromToken(typeToken));
        this.state.syntheticIds.put(syntheticId, toReturn);
      }
      return toReturn;
    }
docs: Get-or-create method for synthetic ids.
  
  @see #syntheticIds