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 (2025-09-09 14:29:31 GIT hash 98644f8f651c2101a0730cfe31c5807993b7603b built by fridi@fzen)
head of the list, 1 if it comes directly after head, etc. nil if x is
not in the list.