<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def write_libraries(dir, libraries):
 """Write a list of libraries to disk. Args: dir: Output directory. libraries: List of (filename, library) pairs. """

  files = [open(os.path.join(dir, k), "w") for k, _ in libraries]
  # Document mentioned symbols for all libraries
  for f, (_, v) in zip(files, libraries):
    v.write_markdown_to_file(f)
  # Document symbols that no library mentioned.  We do this after writing
  # out all libraries so that earlier libraries know what later libraries
  # documented.
  for f, (_, v) in zip(files, libraries):
    v.write_other_members(f)
    f.close()