@@ -27,10 +27,11 @@
          pollen/file
          net/uri-codec
          gregor
          txexpr
          racket/list
+         racket/system
          racket/string)
 
 ;; Provides common helper functions used throughout the project
 
 (provide maybe-meta     ; Select from (current-metas) or default value ("") if not available
@@ -37,10 +38,11 @@
          maybe-attr     ; Return an attribute’s value or a default ("") if not available
          here-output-path
          series-noun    ; Retrieve noun-singular from current 'series meta, or ""
          series-title   ; Retrieve title of series in current 'series meta, or ""
          series-pagenode
+         invalidate-series
          make-tag-predicate
          tx-strs
          ymd->english
          ymd->dateformat
          default-authorname
@@ -59,13 +61,15 @@
 (define series-folder "series")
 (define articles-folder "articles")
 
 (define (default-title body-txprs)
   (format "“~a…”" (first-words body-txprs 5)))
+
 
 (define (maybe-meta m [missing ""])
-  (or (select-from-metas m (current-metas)) missing))
+  (cond [(current-metas) (or (select-from-metas m (current-metas)) missing)]
+        [else missing]))
 
 ;; Return the current output path, relative to (current-project-root)
 ;; Similar to the variable 'here' which is only accessible in Pollen templates,
 ;; except this is an actual path, not a string.
 (define (here-output-path)
@@ -94,10 +98,23 @@
 (define (series-title)
   (define series-pnode (series-pagenode)) 
   (case series-pnode
     ['|| ""] ; no series specified
     [else (or (select-from-metas 'title series-pnode) "")]))
+
+;; “Touches” the last-modified date on the current article’s series, if there is one
+
+(define (invalidate-series)
+  (define series-name (maybe-meta 'series #f))
+  (when series-name
+    (define series-file (build-path (current-project-root)
+                                    series-folder
+                                    (format "~a.poly.pm" series-name)))
+    (when (file-exists? series-file)
+      (case (system-type 'os)
+        ['windows (system (format "type nul >> ~a" series-file))]
+        [else (system (format "touch ~a" series-file))]))))
 
 ;; ~~~ Project-wide Pagetrees ~~~
 
 (define (include-in-pagetree folder extension)
   (define (matching-file? f)