reserve X,Z for set;
reserve x,y,z for object;
reserve A,B,C for Subset of X;

theorem Th50:
  for A being set, D being a_partition of A holds
    D = Class EqRelOf PreorderFromPartition(D)
proof
  let A be set, D be a_partition of A;
  ERl D = EqRelOf PreorderFromPartition(D) by Th49;
  hence thesis by Def5;
end;
