theorem thLinPreordEvents:
  e1 <= e2 or e2 <= e1
proof
  the events of DS is strongly_connected;
  then [e1,e2] in the InternalRel of the events of DS or
       [e2,e1] in the InternalRel of the events of DS by RELAT_2:def 7;
  hence e1 <= e2 or e2 <= e1 by ORDERS_2:def 5;
end;
