masarati@aero.polimi.it writes:

Both changes will cause binary incompatibility; the latter will not require to modify custom backends (I've already modified all the existing backends, exploiting in most cases the new feature; in back-bdb there is room for minimal improvement, as DN lookahead is relatively straightforward, so if base/scope is defined, but no filter, some advantages can be expected).

Could you put them at the end of the tool_globals variable instead? Then the binary incompatibility will only be in tool_globals, not in the backend structure. I suppose this variant makes a bit a mess for glue if nothing else - I assume it would need to to save/modify/restore such global parameters. Slap tools could accept these parameters only for backends which which set a flag to say they handle them.

Might even put them in a mostly empty Operation in tool_globals. Some backends may be able to reuse existing Search code that way, and other search-like parameters could be added later if we want. However that introduces incompatibility after all, so maybe that's for RE25. '#define tv_sub_ndn tv_op.o_req_ndn' can handle source code compat.