- monitor PIDprocedure
The calling process starts monitoring the pid process.
Return a monitor reference ref.
If that process exits, either normally or not, the caller process will be informed with a message of the form:
('DOWN ref pid reason)
where reason can be one of the following:
- symbol exited
- the process exited normally
- symbol no-process
- the process didn't exist
- list ('condition c)
- the process exited because of condition c