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 (2026-02-20 12:42:29 GIT hash dbcc3e4b3d2a59f66230b482cff8f0b018178d77 built by fridi@fzen)
head of the list, 1 if it comes directly after head, etc. nil if x is
not in the list.