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IIRC it's difficult to walk macroexpanded code in Common Lisp because you have to deal with lots of implementation-specific special forms.

(Edit) This article has some good info: https://zenodo.org/record/3254669#.YL3-WzYzb2w

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Exactly! If you walk the expanded form you have implementation-specific junk to deal with. Also, it may not be possible to map a form post-expansion to a source location. If you walk the pre-expansion form you’re going to get bad results. There’s no winning here unless you have help from the implementation.

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