Alias structure ctors, to avoid damage from shadowing.

os2.rkt

@@ -36,6 +36,18 @@
 
 	 (struct-out at-meta-level)
 
+	 ;; Aliases of structures themselves, because of shadowing of
+	 ;; constructors/type-names.
+	 (rename-out [topic <topic>])
+	 (rename-out [handlers <handlers>])
+	 (rename-out [transition <transition>])
+	 (rename-out [add-role <add-role>])
+	 (rename-out [delete-role <delete-role>])
+	 (rename-out [send-message <send-message>])
+	 (rename-out [spawn <spawn>])
+	 (rename-out [kill <kill>])
+	 (rename-out [at-meta-level <at-meta-level>])
+
 	 ;; Reexports from unify.rkt for convenience
 	 wild
 	 wild?