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

theorem
  X c= Y \/ Z & X misses Z implies X c= Y
proof
  assume that
A1: X c= Y \/ Z and
A2: X /\ Z = {};
  X /\ (Y \/ Z)= X by A1,Th28;
  then Y /\ X \/ {} = X by A2,Th23;
  hence thesis by Th17;
end;
