def string_of_items(dic):
    """
    Return a string containing all keys of a dictionary, separated by a comma
    (Helper function for structure inlining)
    :param dic: dictionary [key=string: value=string]
    :return: string constructed of the dictionaries' keys
    """
    s = ''
    for k, v in dic.items():
        s += k + ': ' + v + '\n'
    return s