code: def _build_plain_hierarchy(hierarchy, skip_root=False):
      """Expands tree hierarchy representation to parent-child dictionary.
  
      Args:
          hierarchy: labels hierarchy
          skip_root (False): if true skips root from the processing (done for the case when all
              classes under hierarchy are collected under virtual node)
  
      Returns:
          a tuple of:
  
          -   ``keyed_parent``: dictionary of parent - all its children nodes
          -   ``keyed_child``: dictionary of children - all its parent node
          -   ``children``: all children of the current node
      """
      all_children = set([])
      all_keyed_parent = {}
      all_keyed_child = {}
  
      if "Subcategory" in hierarchy:
          for node in hierarchy["Subcategory"]:
              (
                  keyed_parent,
                  keyed_child,
                  children,
              ) = _build_plain_hierarchy(node)
  
              # Update is not done through dict.update() since some children have
              # multiple parents in the hiearchy
              _update_dict(all_keyed_parent, keyed_parent)
              _update_dict(all_keyed_child, keyed_child)
              all_children.update(children)
  
      if not skip_root:
          all_keyed_parent[hierarchy["LabelName"]] = deepcopy(
              all_children
          )
          all_children.add(hierarchy["LabelName"])
          for child, _ in all_keyed_child.items():
              all_keyed_child[child].add(hierarchy["LabelName"])
  
          all_keyed_child[hierarchy["LabelName"]] = set([])
  
      return all_keyed_parent, all_keyed_child, all_children
  
imports: from collections import defaultdict
  from copy import deepcopy
  import numpy
  import fiftyone.core.plots
  import fiftyone.utils.iou
  from .detection import DetectionEvaluation, DetectionEvaluationConfig, DetectionResults
label_desc: Use image node_id 11 for calling a global function with example usage: _build_plain_hierarchy(hierarchy, skip_root) and returns: all_keyed_parent,  all_keyed_child,  all_children