def reduced_params(params, unwanted_keys):
    """Create a copy of params with the selected fields removed.

    Args:
        params (dict): The parameters that define an experiment which may 
        or may not have been run already.

        unwanted_keys (list[string]): The fields to remove.

    Returns:
        dict: A copy of the original parameters minus the selected keys.
    """
    if not isinstance(unwanted_keys, set):
        unwanted_keys = set(unwanted_keys)

    return {k: params[k] for k in params.keys() - unwanted_keys}