theorem Th39:
  X1 <> {} & X2 <> {} & X3 <> {} & X4 <> {} iff [:X1,X2,X3,X4:] <>
  {}
proof
A1: [:[:X1,X2,X3:],X4:] = [:X1,X2,X3,X4:] by ZFMISC_1:def 4;
  X1 <> {} & X2 <> {} & X3 <> {} iff [:X1,X2,X3:] <> {} by Th21;
  hence thesis by A1,ZFMISC_1:90;
end;
