docs: def symlink_plus(orig, new):
      """Create relative symlinks and handle associated biological index files.
      """
      for ext in ["", ".idx", ".gbi", ".tbi", ".bai"]:
          if os.path.exists(orig + ext) and not os.path.lexists(new + ext):
              with chdir(os.path.dirname(new)):
                  os.symlink(os.path.relpath(orig + ext), os.path.basename(new + ext))
      orig_noext = splitext_plus(orig)[0]
      new_noext = splitext_plus(new)[0]
      for sub_ext in [".bai"]:
          if os.path.exists(orig_noext + sub_ext) and not os.path.lexists(new_noext + sub_ext):
              with chdir(os.path.dirname(new_noext)):
                  os.symlink(os.path.relpath(orig_noext + sub_ext), os.path.basename(new_noext + sub_ext))