code: def GET_account_at(self, path_info, account_addr, block_height):
          
          if not check_account_address(account_addr):
              return self._reply_json({'error': 'Invalid address'}, status_code=400)
  
          try:
              block_height = int(block_height)
              assert check_block(block_height)
          except:
              return self._reply_json({'error': 'Invalid block height'}, status_code=400)
  
          blockstackd_url = get_blockstackd_url()
          res = blockstackd_client.get_account_at(account_addr, block_height, hostport=blockstackd_url)
          if json_is_error(res):
              log.error("Failed to list account history for {} at {}: {}".format(account_addr, block_height, res['error']))
              return self._reply_json({'error': 'Failed to get account state for {} at {}'.format(account_addr, block_height)}, status_code=res.get('http_status', 500))
  
          self._reply_json(res)
          return
docs: Get the state(s) of an account at a particular point in history
          Returns [{...}]