A1: [:(X \ A),X:] c= [:X,X:] by ZFMISC_1:95;
[:X,A:] c= [:X,X:] by ZFMISC_1:95;
hence [:(X \ A),X:] \/ [:X,A:] is Subset of [:X,X:] by A1, XBOOLE_1:8; :: thesis: verum