mu-scm: add support for thread-id

Code + doc + test

scm/mu-scm-test.scm

@@ -78,13 +78,16 @@
     (test-equal '("439C1136.90504@euler.org" "4399DD94.5070309@euler.org"
 		  "20051209233303.GA13812@gauss.org" "439B41ED.2080402@euler.org"
 		  "439A1E03.3090604@euler.org" "20051211184308.GB13513@gauss.org")
-      (references msg)))
+      (references msg))
+      (test-equal "439C1136.90504@euler.org" (thread-id msg)))
 
   (let ((msg (car (mfind "subject:\"gcc include search order\""))))
     (test-equal "gcc include search order" (subject msg))
     (test-equal "klub" (header msg "precedence"))
     (test-equal "gcc-help.gcc.gnu.org" (mailing-list msg))
-      (test-equal #f (references msg)))
+    (test-equal #f (references msg))
+    (test-equal "3BE9E6535E3029448670913581E7A1A20D852173@emss35m06.us.lmco.com" (message-id msg))
+    (test-equal "3BE9E6535E3029448670913581E7A1A20D852173@emss35m06.us.lmco.com" (thread-id msg)))
   (test-end "test-message-more"))
 
 (define (test-options)

scm/mu-scm.scm

@@ -41,6 +41,8 @@
   subject
 
   references
+  thread-id
+
   mailing-list
 
   language
@@ -223,6 +225,15 @@ fake-message-id (see impls) are filtered out. If there are no references, return
 #f."
   (find-field message ':references))
 
+(define-method (thread-id (message <message>))
+  "Get the oldest (first) reference for MESSAGE, or message-id if there are none.
+If neither are available, return #f.
+This is method is useful to determine the thread a message is in."
+  (let ((refs (references message)))
+    (if (and refs (not (null? refs)))
+	(car refs)
+	(message-id message))))
+
 (define-method (mailing-list (message <message>))
   "Get the mailing-list id for MESSAGE or #f if not available."
   (find-field message ':list))

scm/mu-scm.texi

@@ -638,6 +638,24 @@ For example:
     439A1E03.3090604@@euler.org" "20051211184308.GB13513@@gauss.org")
 @end lisp
 
+@deffn {Scheme Procedure} thread-id message
+@end deffn
+Get the oldest reference for the message or its message-id if there is none.
+This is useful to identify the thread some message lives in.
+
+For example:
+@lisp
+(thread-id msg)
+=> "439C1136.90504@@euler.org"
+@end lisp
+
+For example:
+@lisp
+(references msg)
+=> ("439C1136.90504@@euler.org" "4399DD94.5070309@@euler.org"
+    "20051209233303.GA13812@@gauss.org" "439B41ED.2080402@@euler.org"
+    439A1E03.3090604@@euler.org" "20051211184308.GB13513@@gauss.org")
+@end lisp
 
 @deffn {Scheme Procedure} mailing-list message
 @end deffn