def group_errors(graph: BELGraph) -> Mapping[str, List[int]]:
warning_summary = defaultdict(list)<EOL>for _, exc, _ in graph.warnings:<EOL><INDENT>warning_summary[str(exc)].append(exc.line_number)<EOL><DEDENT>return dict(warning_summary)<EOL>
Group the errors together for analysis of the most frequent error.

    :return: A dictionary of {error string: list of line numbers}