Fix https://github.com/wesnoth/wesnoth/issues/7923 .when we want to detect an ability with value=0-20 and an ability with add=15 is present, this is detected because the system assigns a value of 0 when 'value' is not present, and so the system matches abilities that do not use 'value' or even no numerical value at all. it is therefore necessary to decide that the absence of an attribute is a non-correspondence to the criteria.
There are questions about how this filter should treat empty values, and the
use cases that we can think of can be implemented in other ways, for example,
if we want to look for big damage boosts, then we could filter for
`add=5-infinity` `[or]` `multiply=2-infinity`.
The simple answer seems to be to remove it from the API before we freeze for 1.18.
There's more discussion in issue 7944.
using overwrite_specials alone means that we have only two possibilities, either one_side is chosen and in this case if the ability used as a weapon carrying the attribute is applied to the unit (apply_to=self), the other abilities are the same type applied also to 'self' not carrying the attribute will be overwritten, but those of the opponent with apply_to=opponent will be kept in the list; or else both_sides is chosen and all abilities of the same type that do not carry the attribute will be overwritten. To be able to use the attribute in abilities like [damage] for example, it is necessary to be able to be even more selective as for a 'charge' type leadership with multiply=2.5 but which must not be combined with the classic charge and without overwriting the aute [damage] as backstab, [overwrite_filter] to only match damage with apply_to=both and active_on=offense is then interesting.
adding priority allows you to select that it ability can use its overwrite_specials attribute while the others can be overwritten in the same way as an ability without the attribute. Finally, this system makes it unnecessary to limit the use of the attribute to abilities used as weapons but also to special weapons.
