reserve x,y for object,X,Y for set;
reserve M for Pnet;

theorem
  the carrier of empty_f_net = {} & the carrier' of empty_f_net = {} &
  Flow empty_f_net = {}
proof
  empty_f_net = PT_net_Str (# {}, {}, {}({},{}), {}({},{}) #)
     by Def1,XBOOLE_1:65;
  hence thesis;
end;
