[elephant-devel] collecting values from virtual subtree?

[Joe Corneli]

In the discussion of how to build a "virtual subtree", 

http://common-lisp.net/pipermail/elephant-devel/2007-April/000949.htm

Ian gave me a tip on how to find all Triples with a certain property:

   [My recipe] will create an index 'triples-first which only indexes
   triples and does so by the value of the first element.  Thus you
   can easily retrieve all triples with the first element eq to 5.

   ELE-TESTS> (map-index (lambda (sk v pk) (print v)) (get-index my- 
   things 'triples-first :value 5)))

Just wanted to point out that I want to *collect* these values -- so I
wrote a function that does this:

(defun match-triples-beginning (beginning)
  (let ((results (list nil)))
    (map-index (lambda (k v pk) 
                 (declare (ignore k pk)) 
                 (setq results (nconc results (list v))))
               (get-index *things* 'triples-beginning)
               :value beginning)
    (cdr results)))

This makes me think that it would be nice to have a `mapcar'-style
function built in for mapping across an index and collecting the
results.  First question is, what would such a function be called!
(Maybe just add a :collect keyword to `map-index'?)