code: def clear_file_cache(filename=None):
      
      global file_cache, file2file_remap, file2file_remap_lines
      if filename is not None:
          if filename in file_cache:
              del file_cache[filename]
              pass
      else:
          file_cache = {}
          file2file_remap = {}
          file2file_remap_lines = {}
          pass
      return
docs: Clear the file cache. If no filename is given clear it entirely.
      if a filename is given, clear just that filename.