theorem thEvTrans:
  e1 <= e2 & e2 <= e3 implies e1 <= e3
proof
  [e1,e2] in the InternalRel of the events of DS &
  [e2,e3] in the InternalRel of the events of DS
  implies
  [e1,e3] in the InternalRel of the events of DS
    by ORDERS_2:def 3, RELAT_2:def 8;
  hence thesis by ORDERS_2:def 5;
end;
