code: def add_position(self, radial=False, chunksize=2**19, chunkslice='bytes',
                       comp_filter=default_compression, overwrite=False,
                       params=dict()):
          
          nparams = self.numeric_params
          num_particles = nparams['np']
  
          name, ncoords, prefix = 'position', 3, 'X-Y-Z'
          if radial:
              name, ncoords, prefix = 'position_rz', 2, 'R-Z'
          title = '%s position trace of each particle' % prefix
          return self.add_trajectory(name, shape=(num_particles, ncoords, 0),
                                     overwrite=overwrite, chunksize=chunksize,
                                     comp_filter=comp_filter,
                                     atom=tables.Float32Atom(),
                                     title=title,
                                     params=params)
docs: Add the `position` array in '/trajectories'.