[erlang-questions] questions about system events

[Xavier Noria]

The section "System Messages" of Cesarini & Vinosky explains there are
three types of system messages:

    {in, Msg}
    {out, Msg, To, State}
    term()

and mentions that the docs up to Erlang 18 also specify

    {in, Msg, From}
    {out, Msg, To}

but they are (were) not used by any standard behaviors.

I have compared this to the current docs (
http://erlang.org/doc/man/sys.html#type-system_event) and see a few things:

* Current docs use the term "system event" rather than "system message". Is
that a change in terminology that happened at some point?

* I guess the API has changed since Erlang 18 and {out, Msg, To, State} is
no longer documented, there's {out, Msg, To} today. Are you supposed to
issue additional calls in order to trace state?

* {in, Msg, From} is still documented, is now used by some standard
behavior?
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