Follow graph with snapshots and recommendations

Build SocialNetwork, a directed "who follows whom" graph, with snapshots that freeze its state.

• add_user(user): raise ValueError if the user already exists.
• follow(follower, followee): raise ValueError if either user doesn't exist. Following yourself, or someone you already follow, does nothing.
• unfollow(follower, followee): raise ValueError if either user doesn't exist; does nothing if there's no such follow.
• is_following(follower, followee) -> bool: the current state; False for unknown users.
• create_snapshot() -> Snapshot: a frozen copy of the graph right now.

Snapshot has is_following(follower, followee) -> bool (False for users it doesn't know). A snapshot never sees follows, unfollows or users that happen after it was created.

net = SocialNetwork()
for u in ("ann", "bob", "cy"):
    net.add_user(u)
net.follow("ann", "bob")
snap = net.create_snapshot()
net.follow("ann", "cy")
net.unfollow("ann", "bob")
snap.is_following("ann", "bob")   # True
snap.is_following("ann", "cy")    # False
net.is_following("ann", "bob")    # False