Execute a sub-process asynchronously.

• The sub-process reads stdin from a lazy list and writes stdout and stderr to a port. These two ports are closed when the stdout and stderr streams are closed.
• The executable is searched for in the current PATH.

Execute a sub-process asynchronously in it's own process group.

• The sub-process reads stdin from a lazy list and writes stdout and stderr to a port. These two ports are closed when the stdout and stderr streams are closed.
• The executable is searched for in the current PATH.
• The process group id is the same as the new process' process id.

Process operators ------------------

Returns the exit status of the given process (group).

• Blocks until the value is available.
• In case of a NoProcId, the availability of the unit argument implements a synchronisation point.
• In case of a process group, the status is ExitSuccess iff the exit status of all processes is ExitSuccess; otherwise, it is the failure status of any of the failed processes (it is, of course, always deterministically the same, which is picked).