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

theorem Th93:
  X \/ Y = (X \+\ Y) \/ X /\ Y
proof
  thus X \/ Y = ((X \ Y) \/ X /\ Y) \/ Y by Th51
    .= (X \ Y) \/ (X /\ Y \/ Y) by Th4
    .= (X \ Y) \/ Y by Th22
    .= (X \ Y) \/ ((Y \ X) \/ (Y /\ X)) by Th51
    .= (X \+\ Y) \/ X /\ Y by Th4;
end;
