This issue proposes new syntax extending sets from #3 to exclude matches. The proposal uses a set difference approach.
One proposed syntax is to deliminate within the set syntax. Using a hyphen, an example would be /X/{* - A}/Y where * would match all fields of X and A would be removed from this set. An alternative proposed syntax is to deliminate outside the set syntax, such as in /X/{*}-{A}/Y, though in this syntax it may not be immediately clear to the user that {*}-{A} acts as a unit.
Commas are union operations as normal from #3 not only for the inclusion side but also the exclusion side in the set difference, like in /X/{A*, B*, C* - A1, ?2, C?*}/Y. This is one aspect of the alternative syntax that may be better as each side of {A*, B*, C*}-{A1, ?2, C?*} in /X/{A*, B*, C*}-{A1, ?2, C?*}/Y is clearly an independent set.
It is unclear at this time how this would interact with #19. Perhaps set differences could apply to not just individual fields but whole arrays of segments. An open question would be whether A/B/C should be removed from /A{/*/C - /B} or not. My intuition says no as you would need to do /A{/*/C - /B/C} to do that.