code: def copyto_at(self, n, src, m):
          
          for k in self.containers:
              self.__dict__[k][n] = src.__dict__[k][m]
docs: Copy to at a given location. 
  
          Parameters
          ----------
          n: int 
              index where to copy 
          src: `ThetaParticles` object
              source
          m: int
              index of the element to be copied
  
          Note
          ----
          Basically, does self[n] <- src[m]