-----
    Brief
    -----
    A sub-function of _filter_keywords function.

    -----------
    Description
    -----------
    Function used for identification when a list of keywords contains invalid keywords not present
    in the valid list.

    ----------
    Parameters
    ----------
    list_keys : list
        List of keywords that must be verified, i.e., all the inputs needs to be inside valid_keys
        in order to a True boolean be returned.

    valid_keys : list
        List of valid keywords.

    Returns
    -------
    out : boolean, list
        Boolean indicating if all the inserted keywords are valid. If true a list with invalid
        keywords will be returned.

def _inv_key(list_keys, valid_keys):
    """
    -----
    Brief
    -----
    A sub-function of _filter_keywords function.

    -----------
    Description
    -----------
    Function used for identification when a list of keywords contains invalid keywords not present
    in the valid list.

    ----------
    Parameters
    ----------
    list_keys : list
        List of keywords that must be verified, i.e., all the inputs needs to be inside valid_keys
        in order to a True boolean be returned.

    valid_keys : list
        List of valid keywords.

    Returns
    -------
    out : boolean, list
        Boolean indicating if all the inserted keywords are valid. If true a list with invalid
        keywords will be returned.
    """

    inv_keys = []
    bool_out = True
    for i in list_keys:
        if i not in valid_keys:
            bool_out = False
            inv_keys.append(i)

    return bool_out, inv_keys