def collection_2_listed_string(set_of_cities):
    """
    Given a container of strings,
    returns an HTML string where each city has its own line
    This function will be used to help format and build the email text body in the build_email_message function

    Note: returned cities should be in alphabetical order
    """

    # Early exit if the set is empty
    if not set_of_cities:
        return 'None Available' + '<br>'

    string = ""  # Initialize empty string

    sort = sorted(set_of_cities)  # Sorts the set (converts to a list)

    for city in sort:  # Loop over each city in the set
        string += city + '<br>'  # Append the city to the string

    return string  # Returns the string