theorem Th29:
  x in X implies Ext({X},x,y) = { X\/{y} }
proof
  assume
A1: x in X;
  thus Ext({X},x,y) c= { X\/{y} }
  proof
    let a be object;
    assume a in Ext({X},x,y);
    then per cases by XBOOLE_0:def 3;
    suppose a in {A\/{y} where A is Element of {X}: x in A};
      then consider A be Element of {X} such that
A2:     a=A\/{y} & x in A;
      A=X by TARSKI:def 1;
      hence thesis by A2,TARSKI:def 1;
    end;
    suppose a in {A where A is Element of {X}: not x in A & A in {X}};
      then ex A be Element of {X} st a=A & not x in A & A in {X};
      hence thesis by A1,TARSKI:def 1;
    end;
  end;
  let a be object;
  assume a in { X\/{y} };
  then a=X\/{y} & X in {X} by TARSKI:def 1;
  then a in {A\/{y} where A is Element of {X}: x in A} by A1;
  hence thesis by XBOOLE_0:def 3;
end;
