code: func (l *State) PushThread() bool {
  	l.apiPush(l)
  	return l.global.mainThread == l
  }
docs: // PushThread pushes the thread l onto the stack.  It returns true if l is
  // the main thread of its state.
  //
  // http://www.lua.org/manual/5.2/manual.html#lua_pushthread