theorem Th34:
  union Rank A c= Rank A
proof
  let x be object;
  assume x in union Rank A;
  then consider X such that
A1: x in X and
A2: X in Rank A by TARSKI:def 4;
 X c= Rank A by A2,ORDINAL1:def 2;
  hence thesis by A1;
end;
