reserve W,X,Y,Z for set,
  f,g for Function,
  a,x,y,z for set;
reserve u,v for Element of Tarski-Class(X),
  A,B,C for Ordinal,
  L for Sequence;

theorem Th49:
  X is epsilon-transitive & Y is epsilon-transitive implies
  X \/ Y is epsilon-transitive
proof
  assume that
A1: ( Z in X implies Z c= X) and
A2: ( Z in Y implies Z c= Y);
  let Z;
  assume Z in X \/ Y;
then  Z in X or Z in Y by XBOOLE_0:def 3;
then A3: Z c= X or Z c= Y by A1,A2;
 X c= X \/ Y & Y c= X \/ Y by XBOOLE_1:7;
  hence thesis by A3;
end;
