code: public static Map<Class, WriteHandler<?, ?>> writeHandlerMap(Map<Class, WriteHandler<?, ?>> customHandlers) {
          return new WriteHandlerMap(customHandlers);
      }
docs: Creates a read-only Map of Class to WriteHandler containing
  default WriteHandlers with customHandlers merged in. Use this
  to build the collection of write handlers once, and pass it to
  repeated calls to TransitFactory.reader. This can be more
  efficient than repeatedly passing a map of custom handlers to
  TransitFactory.writer, which then merges them with the default
  handlers and/or looks them up in a cache each invocation.
  @param customHandlers a map of custom WriteHandler to use in addition
  or in place of the default WriteHandler
  @return a WriteHandlerMap