def _get_nominal_integer_dict(nominal_vals):
    """Convert nominal values in integers, starting at 0.
    
    Args:
        nominal_vals (pd.Series): A series.
    
    Returns:
        dict: An dictionary with numeric values.
    
    """
    d = {}
    for val in nominal_vals:
        if val not in d:
            current_max = max(d.values()) if len(d) > 0 else -1
            d[val] = current_max + 1
    return d