Symmetric difference, disjunctive union, XOR - or whatever you call it - is a set operation where the resulting set is composed of elements which are present in either set, but not in both. The corresponding python operator is ^=.

This operation would lower the required patches for "replacement" changes such as the replacement of a move ability.

Without XOR (2 patches):

PatchRemove<SomeUnit>(..):
    abilities -= {OldMove}

PatchAdd<SomeUnit>(..):
    abilities += {NewMove}

With XOR (1 patch):

PatchXOR<SomeUnit>(..)
    # OldMove will be removed because it is in both sets
    abilities ^= {OldMove, NewMove}