code: def __call__(self, x: "Renderable"):
      stack = RQStack()
      builder = []
  
      while x is not None or stack.non_empty():
          if x is not None:
              head = x.render_head(self)
              if head != "":
                  builder.append(x.render_head(self))
              stack.push(
                  RenderableQueue(
                      x.render_children(self), x.render_tail(self)
                  )
              )
              x = None
          else:
              top = stack.peek()
              if top.exhausted():
                  stack.pop()
                  builder.append(top.tail)
              else:
                  builder.append(" ")
                  x = top.pop()
  
      return "".join(builder)
  
imports: from hail import ir
  import abc
  from typing import Sequence, MutableSequence, List, Set, Dict, Optional
  from collections import namedtuple
label_desc: Use image node_id 2 for calling the PlainRenderer obj's underlying member method code with example usage: obj.__call__(x) and returns: str