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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def clean_meta(meta, including_info=False, logger=None):
 """ Clean meta dict. Optionally log changes using the given logger. @param logger: If given, a callable accepting a string message. @return: Set of keys removed from C{meta}. """

    modified = set()

    for key in meta.keys():
        if [key] not in METAFILE_STD_KEYS:
            if logger:
                logger("Removing key %r..." % (key,))
            del meta[key]
            modified.add(key)

    if including_info:
        for key in meta["info"].keys():
            if ["info", key] not in METAFILE_STD_KEYS:
                if logger:
                    logger("Removing key %r..." % ("info." + key,))
                del meta["info"][key]
                modified.add("info." + key)

        for idx, entry in enumerate(meta["info"].get("files", [])):
            for key in entry.keys():
                if ["info", "files", key] not in METAFILE_STD_KEYS:
                    if logger:
                        logger("Removing key %r from file #%d..." % (key, idx + 1))
                    del entry[key]
                    modified.add("info.files." + key)

            # Remove crap that certain PHP software puts in paths
            entry["path"] = [i for i in entry["path"] if i]

    return modified