reserve S for non empty non void ManySortedSign;
reserve X for non-empty ManySortedSet of S;
reserve x,y,z for set, i,j for Nat;

theorem Th9:
  for X being non empty constituted-DTrees set holds X c= Subtrees X
  proof
    let X be non empty constituted-DTrees set;
    let x be object; assume x in X;
    then reconsider x as Element of X;
    reconsider p = {} as Element of dom x by TREES_1:22;
    x = x|p by TREES_9:1;
    hence thesis;
  end;
