diff --git a/minimart/core.rkt b/minimart/core.rkt
index 3727c01..3c9d3ae 100644
--- a/minimart/core.rkt
+++ b/minimart/core.rkt
@@ -21,6 +21,7 @@
 	 (struct-out capture)
 	 pretty-print-matcher
 	 matcher-key-set
+	 matcher-key-set/single
 	 projection->pattern
 
 	 sub
diff --git a/minimart/route.rkt b/minimart/route.rkt
index 117ec44..e557db4 100644
--- a/minimart/route.rkt
+++ b/minimart/route.rkt
@@ -28,6 +28,7 @@
 	 projection->pattern
 	 matcher-project
 	 matcher-key-set
+	 matcher-key-set/single
 	 pretty-print-matcher
 	 matcher->jsexpr
 	 jsexpr->matcher
@@ -690,6 +691,12 @@
     (lambda (m)
       (walk-seq m (lambda (vss vsk) vss)))))
 
+;; Matcher → (Option (Setof Value))
+;; As matcher-key-set, but extracts just the first captured subvalue.
+(define (matcher-key-set/single m)
+  (define vss (matcher-key-set m))
+  (and vss (for/set [(vs (in-set vss))] (car vs))))
+
 (define (struct-type-name st)
   (define-values (name x2 x3 x4 x5 x6 x7 x8) (struct-type-info st))
   name)