<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _add_open_file(self, file_obj):
 """Add file_obj to the list of open files on the filesystem. Used internally to manage open files. The position in the open_files array is the file descriptor number. Args: file_obj: File object to be added to open files list. Returns: File descriptor number for the file object. """

        if self._free_fd_heap:
            open_fd = heapq.heappop(self._free_fd_heap)
            self.open_files[open_fd] = [file_obj]
            return open_fd

        self.open_files.append([file_obj])
        return len(self.open_files) - 1