If the fileId argument is given then it should normally
refer to a process pipeline created with the open command.
In this case the pid command will return a list whose elements
are the process identifiers of all the processes in the pipeline,
in order.
The list will be empty if fileId refers to an open file
that isn't a process pipeline.
If no fileId argument is given then pid returns the process
identifier of the current process.
All process identifiers are returned as decimal strings.