[API] gnc:collector+ and gnc:collector- for collector arithmetic

(gnc:collector+ ...) equivalent to (+ ...)
(gnc:collector- ...) equivalent to (- ...) and will also handle
single-argument sign negation.

gnucash/report/report-system/report-system.scm

@@ -693,6 +693,8 @@
 (export gnc:make-value-collector)
 (export gnc:make-number-collector)      ;deprecated
 (export gnc:make-commodity-collector)
+(export gnc:collector+)
+(export gnc:collector-)
 (export gnc:commodity-collector-get-negated)
 (export gnc:commodity-collectorlist-get-merged) ;deprecated
 (export gnc-commodity-collector-commodity-count)

gnucash/report/report-system/report-utilities.scm

@@ -390,6 +390,25 @@ construct gnc:make-gnc-monetary and use gnc:monetary->string instead.")
 (define (gnc-commodity-collector-allzero? collector)
   (every zero? (map cdr (collector 'format cons #f))))
 
+;; (gnc:collector+ collectors ...) equiv to (+ collectors ...) and
+;; outputs: a collector
+(define (gnc:collector+ . collectors)
+  (let ((res (gnc:make-commodity-collector)))
+    (for-each (lambda (coll) (res 'merge coll #f)) collectors)
+    res))
+
+;; (gnc:collectors- collectors ...) equiv to (- collectors ...), can
+;; also negate single-argument collector. outputs collector
+(define gnc:collector-
+  (case-lambda
+    (() (error "gnc:collector- needs at least 1 collector argument"))
+    ((coll) (gnc:collector- (gnc:make-commodity-collector) coll))
+    ((coll . rest)
+     (let ((res (gnc:make-commodity-collector)))
+       (res 'merge coll #f)
+       (res 'minusmerge (apply gnc:collector+ rest) #f)
+       res))))
+
 ;; add any number of gnc-monetary objects into a commodity-collector
 ;; usage: (gnc:monetaries-add monetary1 monetary2 ...)
 ;; output: a commodity-collector object

gnucash/report/report-system/test/test-report-utilities.scm

@@ -213,6 +213,21 @@
         (gnc:make-gnc-monetary USD 25)
         (coll-A 'getmonetary USD #f))
 
+      (test-equal "gnc:collector+"
+        '(("USD" . 50) ("GBP" . -20))
+        (collector->list
+         (gnc:collector+ coll-A coll-A coll-B)))
+
+      (test-equal "gnc:collector- 1 arg"
+        '(("GBP" . 20) ("USD" . -25))
+        (collector->list
+         (gnc:collector- coll-A)))
+
+      (test-equal "gnc:collector- 3 args"
+        '(("USD" . 25) ("GBP" . -60))
+        (collector->list
+         (gnc:collector- coll-A coll-B coll-B)))
+
       (test-equal "gnc:commodity-collector-get-negated"
         '(("USD" . -25) ("GBP" . 20))
         (collector->list