code: def save(self, X=None, w=None, A=None):
          
          self.X.append(X)
          self.wgt.append(w)
          self.A.append(A)
docs: Save one "page" of history at a given time. 
  
          .. note:: 
              This method is used internally by `SMC` to store the state of the
              particle system at each time t. In most cases, users should not
              have to call this method directly.