编写一个程序用单链表存储多项式，并实现两个一元多项式A与B相加的函数。A，B刚开始指数是递增有序的，A与B之和按降序排列。

为了编写一个程序，我们可以首先定义一个`Node`类表示多项式的项，包括系数和指数。然后创建一个`PolynomialList`类用于管理单链表结构，其中包含一个头部节点和相关的操作方法，如添加项、合并两个多项式等。

class Node:
    def __init__(self, coefficient=0, exponent=0):
        self.coefficient = coefficient
        self.exponent = exponent
        self.next = None

class PolynomialList:
    def __init__(self):
        self.head = None

    # 添加项到链表
    def add_term(self, coefficient, exponent):
        new_node = Node(coefficient, exponent)
        if not self.head:
            self.head = new_node
        else:
            current = self.head
            while current.next and current.next.exponent >= exponent:
                current = current.next
            new_node.next = current.next
            current.next = new_node

    # 合并两个多项式
    def merge_polynomials(self, other):
        combined_list = PolynomialList()
        current_a = self.head
        current_b = other.head
        while current_a and current_b:
            a_coeff = current_a.coefficient
            b_coeff = current_b.coefficient

            if current_a.exponent > current_b.exponent:
                combined_list.add_term(a_coeff, current_a.exponent)
                current_a = current_a.next
            elif current_a.exponent < current_b.exponent:
                combined_list.add_term(b_coeff, current_b.exponent)
                current_b = current_b.next
            else:  # 当两项指数相同时，取较大系数
                combined_list.add_term(max(a_coeff, b_coeff), current_a.exponent)
                current_a = current_a.next
                current_b = current_b.next

        # 如果还有剩余的项，则逐个添加
        while current_a:
            combined_list.add_term(current_a.coefficient, current_a.exponent)
            current_a = current_a.next
        while current_b:
            combined_list.add_term(current_b.coefficient, current_b.exponent)
            current_b = current_b.next
        return combined_list

# 示例：
polynomial_A = PolynomialList()
polynomial_B = PolynomialList()

# 分别为A和B添加项...
# ...

sum_poly = polynomial_A.merge_polynomials(polynomial_B)

这个程序首先定义了单链表中的节点，并实现了多项式链表的基本操作。`merge_polynomials`函数负责合并两个多项式，按照题目要求将结果按降序排列。你可以根据实际需求，补充添加项到`polynomial_A`和`polynomial_B`的具体代码。