code: def _remove_closest_particle(self, p):
          
          #1. find closest pos:
          dp = self.pos - p
          dist2 = (dp*dp).sum(axis=1)
          ind = dist2.argmin()
          rp = self.pos[ind].copy()
          #2. delete
          self.pos = np.delete(self.pos, ind, axis=0)
          return rp
docs: removes the closest particle in self.pos to ``p``