mk: rembero / assoco / nth-o — more list relations

rembero (remove-first) uses nafc to gate the skip-element clause so
the result is well-defined on ground lists. assoco is alist lookup —
runs forward (key -> value) and backward (find keys with a given
value). nth-o uses Peano-encoded indices into a list, mirroring lengtho.

13 new tests, 266/266 cumulative.

plans/minikanren-on-sx.md

@@ -173,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **rembero / assoco / nth-o**: more standard list relations.
+  rembero removes the first occurrence (uses `nafc (== a x)` to gate the
+  skip clause, so it's well-defined on ground lists). assoco is alist
+  lookup — works forward (key → value) and backward (value → key). nth-o
+  uses Peano indices into a list. 13 new tests, 266/266 cumulative.
 - **2026-05-08** — **flatteno**: nested-list flattener via conde + appendo.
   Atom-ness is detected via `nafc (nullo ...) + nafc (pairo ...)` so the
   third clause only fires when the input is a ground non-list. Works for