datalog: findall aggregate (159/159)

(findall L V Goal) — bind L to the distinct V values for which Goal
holds, or the empty list when none. One-line addition to
dl-do-aggregate that returns the unreduced list. Tests cover EDB,
derived relation, and empty cases.

Useful for "give me all the X such that ..." queries without
scalar reduction.

lib/datalog/aggregates.sx

@@ -19,7 +19,7 @@
 ;;
 ;; Empty input: count → 0, sum → 0, min/max → no binding (rule fails).
 
-(define dl-aggregate-rels (list "count" "sum" "min" "max"))
+(define dl-aggregate-rels (list "count" "sum" "min" "max" "findall"))
 
 (define
   dl-aggregate?
@@ -43,6 +43,7 @@
     (cond
       ((= op "count") (len vals))
       ((= op "sum") (dl-sum-vals vals 0))
+      ((= op "findall") vals)
       ((= op "min")
         (cond
           ((= (len vals) 0) :empty)