- the current output list. It should return one of the candidate items. If
- PICK is omitted then an arbitrary choice is made.
+ the current output list. It should return one of the candidate items.
+ The order of the candidates in the list given to the PICK function
+ reflects their order in the input LISTS: item A will precede item B in the
+ candidates list if and only if an occurrence of A appears in an earlier
+ input list than any occurrence of item B. (This completely determines the
+ order of the candidates: it is not possible that two candidates appear in
+ the same input list would resolve the ambiguity between them.) If PICK is
+ omitted then the item chosen is the one appearing in the earliest of the
+ input lists: i.e., effectively, the default PICK function is
+
+ (lambda (candidates output-so-far)
+ (declare (ignore output-so-far))
+ (car candidates))