reserve m for Cardinal,
  A,B,C for Ordinal,
  x,y,z,X,Y,Z,W for set,
  f for Function;
reserve f,g for Function,
  L for Sequence,
  F for Cardinal-Function;
reserve U1,U2,U for Universe;

theorem Th60:
  X in U & Y in U implies X \/ Y in U & X /\ Y in U & X \ Y in U & X \+\ Y in U
proof
  assume that
A1: X in U and
A2: Y in U;
A3: union {X,Y} = X \/ Y by ZFMISC_1:75;
A4: meet {X,Y} = X /\ Y by SETFAM_1:11;
  {X,Y} in U by A1,A2,Th2;
  hence
A5: X \/ Y in U & X /\ Y in U by A3,A4,Th59;
  X \+\ Y = (X \/ Y)\(X /\ Y) by XBOOLE_1:101;
  hence thesis by A1,A5,CLASSES1:def 1;
end;
