support interface to foreign objects that have a backtrack handler.

include/yap_structs.h

@@ -204,6 +204,14 @@ typedef int  (*YAP_agc_hook)(void *_Atom);
 
 typedef void  (*YAP_halt_hook)(int exit_code, void *closure);
 
+typedef int YAP_opaque_tag_t;
+
+typedef int (*YAP_Opaque_CallOnFail)(void *);
+
+typedef struct YAP_opaque_handler_struct {
+  YAP_Opaque_CallOnFail fail_handler;
+} YAP_opaque_handler_t;
+
 /********* execution mode ***********************/
 
 typedef enum