def _prefix_keys(results: dict, prefix: str) -> dict:
    """
    Add a prefix to existing keys

    Args:
        results (dict): The dictionary of results
        prefix (str): A string to prefix each key with

    Returns:
        dict: The result dictionary with prefixed keys.
    """
    prefixed = {}
    for key, val in results.items():
        prefixed[f'{prefix}_{key}'] = val
    return prefixed