Add engio for character animation

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.