It would be nice to be able to use the tests of the engine used to detect collisions in the function custom_entity:add_collision_test(collision_test, callback).

To detect if two given custom entities are in collision (relative to some collision test) at a precise moment, it is not a good idea to add a collision test since you need to check the collision just once and you know when. It would be better to call the corresponding collision test function.

This is not possible yet, I think, except for the function entity:overlaps(other_entity), which is the test function used in the method custom_entity:add_collision_test("overlapping", callback). But, for instance, we do not have a method like entity:is_facing(other_entity), etc.

To make the syntax simpler, I would suggest to have a function like: entity:has_collision(collision_test, other_entity), or something similar, where collision_test is a string representing one of the collision test functions used by the engine (which are not directly accesible at present), like "facing", "touching", "containing" and "sprite".