reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem
  X \/ Y = X \+\ Y \+\ X /\ Y
proof
  X /\ Y misses X \+\ Y by Lm4;
  then (X \+\ Y) \ X /\ Y = X \+\ Y & X /\ Y \ (X \+\ Y) = X /\ Y by Th83;
  hence thesis by Th93;
end;
