Problem 2: A + B (100pts)

写一个代表变量a和b之和的算术表达式，来替换掉下面sum函数中的下划线。

def sum(a, b):
    """Compute the sum of a and b.

    Expected result:
    >>> sum(1, 2)
    3
    >>> sum(3, 8)
    11
    """
    return ______