theorem Th20:
  {{}} is Chain of W
proof
 {} in W by TREES_1:22;
  then reconsider S = {{}} as Subset of W by ZFMISC_1:31;
 S is Chain of W
  proof
    let p,q;
    assume that
A1: p in S and
A2: q in S;
 p = {} by A1,TARSKI:def 1;
    hence thesis by A2,TARSKI:def 1;
  end;
  hence thesis;
end;
