iex> {:ok, agent} = Agent.start_link fn -> [] end
{:ok, #PID<0.112.0>}

iex> :sys.trace(agent, true)
:ok

iex> Agent.get_and_update(agent, fn state -> {state, [1 | state]} end)
*DBG* <0.112.0> got call {get_and_update, #Fun<erl_eval.44.12345123} from <0.110.0>
*DBG* <0.112.0> sent [] to <0.110.0>, new state [1]
[]

iex> Agent.get(agent, fn state -> state end)
*DBG* <0.112.0> got call {get, #Fun<erl_eval.44.12345124} from <0.110.0>
*DBG* <0.112.0> sent [1] to <0.110.0>, new state [1]
[1]

iex> :sys.trace(agent, false)
:ok

iex> Agent.get(agent, fn state -> state end)
[1]