code: function LocalEnvironment(env, id) {
  	Environment.call(this, {}, env._fm.$log);
  	this._env = env;
  	this._thread_id = id;
  }
docs: The LocalEnvironment class, which is the same as an environment class, except that it is
  unique to each "thread" in parallel execution chains, similar to thread-local storage.
  It has a pointer to the shared state, so that it may still be accessed.
  @param env The environment variable to use as parent
  @param id The thread id, this is used to track when all threads complete