def add_prefix(dct, prefix, sep="/"):
    """
    Add a prefix to all keys in `dct`.

    :param dct: Input dict
    :type dct: dict
    :param prefix: Prefix
    :type prefix: str
    :param sep: Separator between prefix and key
    :type sep: str
    :return: Dict with prefix prepended to all keys
    :rtype: dict
    """
    return {prefix + sep + key: value for key, value in dct.items()}