Pipe cleaner: ensuring the correctness of XProc pipelines

As users begin to explore using XProc 3.x pipelines, and migrate existing 1.0 pipelines to 3.x, they naturally have questions about how to tell if a pipeline will work and will produce the correct result. This breaks down, broadly, into four categories: is the pipeline written correctly: is it syntactically valid; is the pipeline written correctly: is logically valid; does it do what the author intended: does it produce the correct results; and if it doesn’t, how can the author figure out why?