code: def to_csv(self, file):
          
          file.write("description,time,value\n")
          for traj in self:
              for t,v in traj:
                  file.write("%s,%f,%f\n"% (traj.description.symbol, t, v))
docs: Write all the trajectories of a collection to a csv file with the headers 'description', 'time' and 'value'.
  
          :param file: a file object to write to
          :type file: :class:`file`
          :return: