def belongs_to_single_index(param_parts):
    """
    Assert param belong to single index group.

    Parameter
    ---------
    param_parts : list of str
        The parameter name parts after `str.split("_")`.

    Returns
    -------
    boolean
        True if the parameter name follows the rules of single index
        expandable parameters.

        Rules:
        - param names have more than 2 parts
        - the last part is a digit
        - the part before last is not a digit
    """
    return \
        len(param_parts) > 2 \
        and param_parts[-1].isdigit() \
        and not param_parts[-2].isdigit() \
        and not param_parts[0].startswith("mol")