Use table for member information, fix a few design mistakes in period/month display.

month.scm

@@ -87,14 +87,16 @@
 
  ;; Formats (valid) month as YYYY-MM string
  (define (month->string M)
-   (if (month-valid? M)
-       (let ((y (car M))
-	     (m (cadr M)))
-	 (sprintf "~A-~A~A"
-		  y
-		  (if (< m 10) "0" "")
-		  m))
-       (error 'string->month "Invalid month" M)))
+   (if M
+       (if (month-valid? M)
+	   (let ((y (car M))
+		 (m (cadr M)))
+	     (sprintf "~A-~A~A"
+		      y
+		      (if (< m 10) "0" "")
+		      m))
+	   (error 'string->month "Invalid month" M))
+       "____-__"))
 
  ;; Returns true if both arguments are a valid month and are equal
  (define (month=? m n)