+/*
+ * slap_response_play()
+ *
+ * plays the callback list; rationale: a callback can
+ * - remove itself from the list, by setting op->o_callback = NULL;
+ * malloc()'ed callbacks should free themselves from inside the
+ * sc_response() function.
+ * - replace itself with another (list of) callback(s), by setting
+ * op->o_callback = a new (list of) callback(s); in this case, it
+ * is the callback's responsibility to to append existing subsequent
+ * callbacks to the end of the list that is passed to the sc_response()
+ * function.
+ * - modify the list of subsequent callbacks by modifying the value
+ * of the sc_next field from inside the sc_response() function; this
+ * case does not require any handling from inside slap_response_play()
+ *
+ * To stop execution of the playlist, the sc_response() function must return
+ * a value different from SLAP_SC_CONTINUE.
+ *
+ * The same applies to slap_cleanup_play(); only, there is no means to stop
+ * execution of the playlist, since all cleanup functions must be called.
+ */