from typing import Set


def add_multi_value(prev, new) -> Set:
    """Add a  value to a set or create a new set with prev and new.

    Parameters
    ----------
    prev
        Existing value.
    new
        New value, that should be added.

    Returns
    -------
    Set
        Set containing the previous and new elements.
    """
    if not isinstance(prev, set):
        prev = {prev}
    prev.add(new)
    return prev