theorem Th21:
  X1 <> {} & X2 <> {} & X3 <> {} iff [:X1,X2,X3:] <> {}
proof
A1: X1 <> {} & X2 <> {} iff [:X1,X2:] <> {} by ZFMISC_1:90;
  [:[:X1,X2:],X3:] = [:X1,X2,X3:] by ZFMISC_1:def 3;
  hence thesis by A1,ZFMISC_1:90;
end;
