@pimms.param<EOL><INDENT>def custom_directory(d):<DEDENT>
if d is None: return None<EOL>if not pimms.is_str(d): raise ValueError('<STR_LIT>')<EOL>else: return d<EOL>
dataset.custom_directory is None if no custom directory was provided for the given dataset;
  otherwise it is the provided custom directory.