diff --git a/luprex/cpp/core/invocation.hpp b/luprex/cpp/core/invocation.hpp
index 07303f13..0506382f 100644
--- a/luprex/cpp/core/invocation.hpp
+++ b/luprex/cpp/core/invocation.hpp
@@ -12,10 +12,20 @@
 //
 //   Nothing.
 //
-// KIND_PLAN:
+// KIND_CHOOSE:
 //
-//   Name of a callback function, in plaintext.
+//   Name of a callback function, in plaintext.  The function name
+//   is sanity checked.  First, we ensure that it begins with the
+//   prefix "cb_."  Then we regenerate the gui, and verify that the
+//   gui has the specified callback as an option.
 //
+// KIND_ENGIO:
+//
+//   First, a function name is read from the datapack.  The function
+//   name must be a lua function inside class "engio".  The function
+//   is called with arguments: actor, place, and then additional
+//   arguments of simple dynamic type read from the datapack.
+// 
 // KIND_LUA:
 //
 //   A block of lua source code, in plaintext.
diff --git a/luprex/lua/login.lua b/luprex/lua/login.lua
index 5e5631c9..6ef8285b 100644
--- a/luprex/lua/login.lua
+++ b/luprex/lua/login.lua
@@ -58,3 +58,8 @@ function engio.myfunction(actor, place, a, b, c)
     pprint("B:", b)
     pprint("C:", c)
     end
+
+function engio.moveto(actor, place, action, xyz)
+    -- todo: sanity check the action and xyz.
+    tangible.animate(actor, nil, {action=action, xyz=xyz})
+    end