A ring object created by make-ring.
A non-negative integer.
A Lisp object.
A function designator for a function of two arguments.
position-in-ring finds in the ring ring the first element that matches item and returns its index, or
nil if there is no match. The search starts from index index and proceeds "backward" up to the length of the ring (its current number of elements). In other words, it tests all the elements that would be returned by ring-ref with indices index, index+1, ... , length-1. It does not wrap around, so elements between indices 0 and index are not tested.
position-in-ring-forward does the same except that it searches from index "forward" to the insertion point. In other words, it tests the elements that would be returned by ring-ref with indices index, index-1, ..., 0.
In LispWorks 6.1 and earlier versions, these functions are called
find-in-ring-forward. They have been renamed to match the Common Lisp convention that a function returning an index is named
position-*. The old names are retained for backwards compatibility, but are deprecated.
LispWorks User Guide and Reference Manual - 20 Sep 2017