reserve p,q,r for FinSequence;
reserve u,v,x,y,y1,y2,z for object, A,D,X,Y for set;
reserve i,j,k,l,m,n for Nat;

theorem
  Seg(k + 1) \ Seg k = {k + 1}
proof
A1: Seg(k + 1) = Seg k \/ {k + 1} by FINSEQ_1:9;
  Seg k misses {k + 1} by Th14;
  hence thesis by A1,XBOOLE_1:88;
end;
