* guile: add mu:count to (mu stats)

guile/mu/stats.scm

@@ -23,6 +23,7 @@
   :use-module (ice-9 i18n)
   :use-module (ice-9 r5rs)
   :export ( mu:tabulate
+	    mu:count
 	    mu:average
 	    mu:stddev
 	    mu:correl
@@ -49,6 +50,17 @@ get back a list like
       expr)
     table))
 
+
+(define* (mu:count #:optional (expr #t))
+  "Count the number of messages matching EXPR. If EXPR is not
+provided, match /all/ messages."
+  (let ((num 0))
+    (mu:for-each-message
+      (lambda (msg) (set! num (1+ num)))
+      expr)
+    num))
+
+
 (define (average lst)
   "Calculate the average of a list LST of numbers, or #f if undefined."
   (if (null? lst)