Allow incomplete ballots.

* When calling 'update-vote, a rank of #f will remove the associated
  alternative from the ranking.

deliberate.scm

@@ -211,8 +211,13 @@
 ;;   A reordered copy of lst
 ;;
 (define (reorder lst ranks)
-  (let ((pairs (zip lst ranks)))
-    (map car (sort pairs (lambda (a b) (< (cadr a) (cadr b)))))))
+  (let* ((pairs (zip lst ranks))
+	 (ranked-pairs
+	  (filter (lambda (pair) (not (eq? #f (cadr pair))))
+		  pairs))
+	 (sorted-pairs
+	   (sort ranked-pairs (lambda (a b) (< (cadr a) (cadr b))))))
+    (map car sorted-pairs)))
 
 
 

test-deliberate.scm

@@ -12,5 +12,5 @@
 (define vote (a-run (spawn ^ranked-choice-vote delib ($ delib 'debug-alternatives))))
 (define ballot (a-run ($ vote 'add-ballot)))
 
-(define (rank312 alts) '(3 1 2))
-(a-run ($ ballot 'update-vote rank312))
+(define (rank31f alts) '(3 1 #f))
+(a-run ($ ballot 'update-vote rank31f))