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Why not more than 4? Another thing CL did "better":

https://www.lispworks.com/documentation/lw50/CLHS/Body/v_deb...

https://www.lispworks.com/documentation/lw50/CLHS/Body/v_ter...

Sure, it is arguably cleaner to explicitly isolate error condition from general process meta-data. So in my OP diagram you can add an errout coming down from the proc-box. I've used this actual pattern in-process to hook up pipelines of active/passive components. However, there is no sense (imo) to propagate the errout of P(n) to P(n+1), so 2-in, 3-out.

p.s. That is pL(n).stderr -> pE.stdin, where pL is the 'business logic' and pE is the system's error processing aspects. I.e. the error processing component's stdin is the stderr of the logical processes (Lp), so there is a uniform process model applicable to both logical and error processing elements of the pipeline.

The issue is how to do this within the limits of line terminal interface (CLI). In code (as in in-process chaining) that aspect is a non-issue.

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