DifferenceBlist( blist1, blist2 )
DifferenceBlist  returns the  asymmetric  set  difference  of the   two
boolean  lists blist1 and blist2, which  must have equal length.  The
asymmetric set difference is a new boolean list such that union[i]
= blist1[i] and not blist2[i].
    gap> blist1 := [ true, true, false, false ];;
    gap> blist2 := [ true, false, true, false ];;
    gap> DifferenceBlist( blist1, blist2 );
    [ false, true, false, false ] 
Note  that DifferenceBlist  is implemented in   terms  of the procedure
SubtractBlist (see SubtractBlist).
GAP 3.4.4