let W be Tree; :: thesis: {{}} is Chain of W
{} 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 be FinSequence of NAT ; :: according to TREES_2:def 3 :: thesis: for q being FinSequence of NAT st p in S & q in S holds
p,q are_c=-comparable
let q be FinSequence of NAT ; :: thesis: ( p in S & q in S implies p,q are_c=-comparable )
assume that
A1: p in S and
A2: q in S ; :: thesis: p,q are_c=-comparable
p = {} by A1, TARSKI:def 1;
hence p,q are_c=-comparable by A2, TARSKI:def 1; :: thesis: verum

end;

hence {{}} is Chain of W ; :: thesis: verum