determine the index of element x within this list. 0 if x is at the head of the list, 1 if it comes directly after head, etc. nil if x is not in the list.
0.095dev (2025-08-15 12:02:22 GIT hash 301b5b75e77076d091b38f555473f9f0e31e5b5c built by fridi@fzen)
head of the list, 1 if it comes directly after head, etc. nil if x is
not in the list.