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 (2025-10-09 18:01:25 GIT hash 4826e516431c193e428991dd546a46472973d8d4 built by fridi@fzen)
head of the list, 1 if it comes directly after head, etc. nil if x is
not in the list.