> LOOP is not extensible The standardized LOOP is not extensible. Each LOOP implementation is extensible. A common extension is to provide new loop iteration p

> LOOP is not extensible The standardized LOOP is not extensible. Each LOOP implementation is extensible. A common extension is to provide new loop iteration paths. LispWorks: CL-USER 7 > (lispworks:defloop (s-element s-elements) elt length sequence t) (S-ELEMENT S-ELEMENTS) Now we can iterate over sequences in a not performance optimized version: CL-USER 8 > (loop for e being each s-element of "hello-world" count e) 11 CL-USER 9 > (loop for e being each s-element of '(hello world) count e) 2 There are portable extensible LOOP implementations. This is then just as non-standard as any self-defined iteration library. https://research.gold.ac.uk/id/eprint/2344/1/sequences-20070301.pdf https://research.gold.ac.uk/id/eprint/2344/1/sequences-20070... gives an example to extend SBCL's LOOP (which is based on the MIT LOOP implementation). There are lots of things in the Common Lisp standard, which are not extensible, but where implementations and libraries provide extensible versions. Example: the CLOS MOP, sequences, hash tables, ...