reserve T for non empty TopSpace,
  A, B for Subset of T,
  F, G for Subset-Family of T;

theorem Th29:
  for F being set holds UNION ({},F) = {}
proof
  let F be set;
  UNION ({},F) c= {}
  proof
    let x be object;
    assume x in UNION ({},F);
    then ex x1, x2 being set st x1 in {} & x2 in F & x = x1 \/ x2 by
SETFAM_1:def 4;
    hence thesis;
  end;
  hence thesis;
end;
