[xswyygqh] Process tree

A process tree or stack trace is a hugely useful tool for understanding function of computer code, or more critically, understanding malfunction.  The canonical example with a process tree is discovering the root cause of what is causing CPU load.

For computer systems not strictly organized as a process tree, provide it as an abstraction.  For example, two independent processes: the first process places a work item into the queue of the second process (which can accept work items from many other processes also).  The work item remains abstractly "owned" by the first process: the subset of work that the second does in working on the work item should be annotated with the first process as an ancestor.