code: def add_particle(self, pos, rad):
          
          rad = listify(rad)
          # add some zero mass particles to the list (same as not having these
          # particles in the image, which is true at this moment)
          inds = np.arange(self.N, self.N+len(rad))
          self.pos = np.vstack([self.pos, pos])
          self.rad = np.hstack([self.rad, np.zeros(len(rad))])
  
          # update the parameters globally
          self.setup_variables()
          self.trigger_parameter_change()
  
          # now request a drawing of the particle plz
          params = self.param_particle_rad(inds)
          self.trigger_update(params, rad)
          return inds
docs: Add a particle or list of particles given by a list of positions and
          radii, both need to be array-like.
  
          Parameters
          ----------
          pos : array-like [N, 3]
              Positions of all new particles
  
          rad : array-like [N]
              Corresponding radii of new particles
  
          Returns
          -------
          inds : N-element numpy.ndarray.
              Indices of the added particles.