To enforce the already existing behavior that atoms
have execute and revert methods (this is already the
de-facto behavior with its subclasses) we might as
well have the atom class be abstract and have methods
execute() and revert() that need to be implemented
in subclasses (which they already are).
Change-Id: Idfc25a7c2b01f674a065e3f467607eba4ab89a26