import itertools

def combine(primary_list: list[str], secondary_list: list[str], separator: str = " ") -> list[str]:
    """
    Combine two lists into a single list with a separator.
    
    Args:
        primary_list (list[str]): The first list.
        secondary_list (list[str]): The second list.
        separator (str, optional): The separator used to join elements. Defaults to a space (" ").

    Returns:
        list[str]: The combined list.
    """
    if not primary_list or not secondary_list:
        return primary_list or secondary_list
    return [f"{item2}{separator}{item1}" for item1, item2 in itertools.product(primary_list, secondary_list)]

def split_number(number: str) -> list[str]:
    """
    Split a number string into parts according to the Indian numbering system.

    Args:
        number (str): The number to split.

    Returns:
        list[str]: The list of constituent parts.
    """
    parts = [number[-3:]] + [number[max(i-2, 0):i] for i in range(len(number)-3, 0, -2)]
    return list(filter(None, parts))