Groups a list of items using the first element in each pair as the item and
    the second element as the groupid.

    Args:
        pair_list (list): list of 2-tuples (item, groupid)

    Returns:
        dict: groupid_to_items: maps a groupid to a list of items

    SeeAlso:
        group_items

def group_pairs(pair_list):
    """
    Groups a list of items using the first element in each pair as the item and
    the second element as the groupid.

    Args:
        pair_list (list): list of 2-tuples (item, groupid)

    Returns:
        dict: groupid_to_items: maps a groupid to a list of items

    SeeAlso:
        group_items
    """
    # Initialize dict of lists
    groupid_to_items = defaultdict(list)
    # Insert each item into the correct group
    for item, groupid in pair_list:
        groupid_to_items[groupid].append(item)
    return groupid_to_items