<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _close_open_file(self, file_des):
 """Remove file object with given descriptor from the list of open files. Sets the entry in open_files to None. Args: file_des: Descriptor of file object to be removed from open files list. """

        self.open_files[file_des] = None
        heapq.heappush(self._free_fd_heap, file_des)