reserve T for non empty RelStr,
  a for Element of T;
reserve a for set;

theorem Th4:
  [#] T = {a} implies T is connected
proof
  reconsider OT = [#] T as non empty set;
  assume
A1: [#] T = {a};
A2: for Z,Z9 be non empty Subset of OT holds not Z misses Z9
  proof
    let Z,Z9 be non empty Subset of OT;
    Z = {a} by A1,ZFMISC_1:33;
    hence thesis by A1,ZFMISC_1:33;
  end;
  [#] T is connected
  by A2;
  hence thesis;
end;
