# Two concurrent add() calls on one conversation

seeded with:  u0, a0
two writers each add one message; both read the conversation before either writes it back
first writer to save: adds a 3-message list, saved
second writer to save: adds a different 3-message list, replaces the first
stored afterwards: [u0, a0, <second writer's message>]   (3 messages, 4 were sent)
the first writer's message is GONE, the second writer's is kept