docstr: append node to children list
  
  Arguments:
      child {AbstractPlan} -- input child node
docstr_tok: ["append","node","to","children","list","Arguments",":","child","{","AbstractPlan","}","--","input","child","node"]
code: def append_child(self, child):
      """append node to children list
  
      Arguments:
          child {AbstractPlan} -- input child node
      """
      self._children.append(child)
  
imports: from abc import ABC, abstractmethod
  from collections import deque
  from typing import Any, List
  from evadb.plan_nodes.types import PlanOprType
label_desc: Use image node_id 2 for calling the AbstractPlan obj's underlying member method code with example usage: obj.append_child(child) without return types