The call index x l where l is a list returns the position number of the first
instance of x in the list, failing if there is none. The indices start at zero,
corresponding to el.
EXAMPLE
# index "j" (explode "abcdefghijklmnopqrstuvwxyz");;
val it : int = 9
This is a sort of inverse to the indexing into a string by el:
# el 9 (explode "abcdefghijklmnopqrstuvwxyz");;
val it : string = "j"