实现深拷贝函数，需要递归遍历整个嵌套的双向链表，对于每个节点进行深拷贝操作，并处理好指针指向。以下是一个示范代码：

class Node:
    def __init__(self, value=None):
        self.value = value
        self.prev = None
        self.next = None
        self.child = None

def copy_node(node):
    # 创建一个新节点
    new_node = Node(value=node.value)
    
    # 拷贝 child 节点
    if node.child:
        new_node.child = copy_list(node.child)

    return new_node

def copy_list(head):
    if not head:
        return None

    new_head = copy_node(head)
    prev_node = new_head
    curr_node = head.next

    while curr_node:
        new_node = copy_node(curr_node)
        new_node.prev = prev_node
        prev_node.next = new_node

        # 如果存在子链表，递归拷贝
        if curr_node.child:
            new_node.child = copy_list(curr_node.child)

        prev_node = new_node
        curr_node = curr_node.next

    return new_head

# 示例
a = Node(1)
b = Node(2)
c = Node(3)
d = Node(4)
e = Node(5)
f = Node(6)
g = Node(7)

a.next = b
b.next = c
b.child = f
c.next = d
c.child = g
d.next = e

new_head = copy_list(a)

上面的代码实现了一个深拷贝函数，能够正确地拷贝嵌套的双向链表，并且不依赖于任何复制库。