test: Add tests to show multi usage

Wasn't able to see examples of this in the docs or tests, so figured it
might be useful to make this capability more discoverable

test/malli/util_test.cljc

@@ -509,7 +509,24 @@
       (is (true? (m/validate
                   (mu/update schema :b identity)
                   {:a 1
-                   :c "a string"}))))))
+                   :c "a string"}))))
+    (testing "Multi schema can update its individual dispatch schemas"
+      (is (mu/equals (-> (m/schema [:multi {:dispatch :type}
+                                    [:sized [:map
+                                             [:type keyword?]
+                                             [:size int?]]]
+                                    [:human [:map
+                                             [:type keyword?]
+                                             [:name string?]
+                                             [:address [:map [:country keyword?]]]]]])
+                         (mu/update :sized mu/select-keys [:size])
+                         (mu/update :human mu/select-keys [:name :address]))
+                     [:multi {:dispatch :type}
+                      [:sized [:map
+                               [:size int?]]]
+                      [:human [:map
+                               [:name string?]
+                               [:address [:map [:country keyword?]]]]]])))))
 
 (deftest assoc-in-test
   (is (mu/equals (mu/assoc-in (m/schema [:vector int?]) [0] string?) [:vector string?]))
@@ -523,7 +540,55 @@
   (is (mu/equals (mu/assoc-in (m/schema [:map]) [:a :b :c :d] int?)
                  [:map [:a [:map [:b [:map [:c [:map [:d int?]]]]]]]]))
   (is (mu/equals (mu/assoc-in (m/schema [:ref {:registry {::a int?, ::b string?}} ::a]) [0] ::b) [:ref {:registry {::a int?, ::b string?}} ::b]))
-  (is (mu/equals (mu/assoc-in (m/schema [:schema int?]) [0] string?) [:schema string?])))
+  (is (mu/equals (mu/assoc-in (m/schema [:schema int?]) [0] string?) [:schema string?]))
+
+  (testing "Multi schema can use assoc-in to introduce new dispatch schemas"
+    (is (mu/equals (-> (m/schema [:multi {:dispatch :type}
+                                  [:sized [:map
+                                           [:type keyword?]
+                                           [:size int?]]]
+                                  [:human [:map
+                                           [:type keyword?]
+                                           [:name string?]
+                                           [:address [:map [:country keyword?]]]]]])
+                       (mu/assoc-in [:robot] [:map
+                                              [:type keyword?]
+                                              [:serial-number string?]
+                                              [:charging-station [:map [:id string?]]]]))
+                   [:multi {:dispatch :type}
+                    [:sized [:map
+                             [:type keyword?]
+                             [:size int?]]]
+                    [:human [:map
+                             [:type keyword?]
+                             [:name string?]
+                             [:address [:map [:country keyword?]]]]]
+                    [:robot [:map
+                             [:type keyword?]
+                             [:serial-number string?]
+                             [:charging-station [:map [:id string?]]]]]])))
+  (testing "Multi schema can use assoc-in to extend existing schemas"
+    (is (mu/equals (-> (m/schema [:multi {:dispatch :type}
+                                  [:sized [:map
+                                           [:type keyword?]
+                                           [:size int?]]]
+                                  [:human [:map
+                                           [:type keyword?]
+                                           [:name string?]
+                                           [:address [:map [:country keyword?]]]]]])
+                       (mu/assoc-in [:sized :unit] string?)
+                       (mu/assoc-in [:human :address :state] string?))
+                   [:multi {:dispatch :type}
+                    [:sized [:map
+                             [:type keyword?]
+                             [:size int?]
+                             [:unit string?]]]
+                    [:human [:map
+                             [:type keyword?]
+                             [:name string?]
+                             [:address [:map
+                                        [:country keyword?]
+                                        [:state string?]]]]]]))))
 
 (deftest update-in-test
   (is (mu/equals (mu/update-in (m/schema [:vector int?]) [0] (constantly string?)) [:vector string?]))