from functools import reduce


def sum(l):
    """
        Returns the sum of the items in the container class.
        This is more general than the build-in 'sum' function, because it is not specific for numbers.
        This function uses the '+' operator repeatedly on the items in the contrainer class.
        For example, if each item is a list, this will return the concatenation of all of them
    """
    return reduce(lambda x,y: x+y, l)