diff --git "a/resources/espa\303\261ol/lexicon/rules.edn" "b/resources/espa\303\261ol/lexicon/rules.edn"
index f87c86816..8fe38b15b 100644
--- "a/resources/espa\303\261ol/lexicon/rules.edn"
+++ "b/resources/espa\303\261ol/lexicon/rules.edn"
@@ -9,8 +9,8 @@
        :boot-stem? true}
   :then [{:exceptions [{:infl :present
                         :agr {:person :1st :number :sing}
-                        :surface #(str (u/get-in % [:stems :boot]) "o")
-                        }
+                        :surface {:prefix [:stems :boot]
+                                  :suffix "o"}}
                        ;; TODO: other numbers and persons.
                        ]}]}
 
diff --git a/src/menard/lexiconfn.cljc b/src/menard/lexiconfn.cljc
index e9c5eecfd..e1fbb47f1 100644
--- a/src/menard/lexiconfn.cljc
+++ b/src/menard/lexiconfn.cljc
@@ -44,12 +44,15 @@
           (merge consequent
                  {:exceptions (map (fn [exception]
                                      (let [surface (:surface exception)]
-                                       (cond (fn? surface)
-                                             ;; TODO: lock down what can be evaluated 
-                                             (merge exception
-                                                    {:surface (surface lexeme)})
-                                             :else
-                                             exception)))
+                                       (cond
+                                         (map? surface)
+                                         (merge exception
+                                                {:surface (str (u/get-in lexeme (:prefix surface))
+                                                               (:suffix surface))})
+                                         (string? surface)
+                                         exception
+                                         :else
+                                         (exception "Cannot find surface form given intermediate surface value: " surface))))
                                    (:exceptions consequent))})]
       retval)
     consequent))