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.
0.094dev (2025-07-10 15:41:47 GIT hash eafa42ae743de61b62f958948d7abb5fada7756f built by fridi@fzen)
head of the list, 1 if it comes directly after head, etc. nil if x is
not in the list.