code: def box_and_fill_missing(
      a: TwoLevelMapping,
      b: TwoLevelMapping,
  ) -> tuple[TwoLevelMappingToBox, TwoLevelMappingToBox]:
      """Returns boxed two level mappings with the same structure.
  
      It is assumed that ``a`` is a subset of ``b``.
  
      Args:
        a: A two level mapping (e.g. Haiku parameters or state).
        b: A two level mapping (e.g. Haiku parameters or state).
  
      Returns:
        A pair of two level mappings with ``Box`` wrapped leaves (suitable for use
        with ``jax.tree_util.tree_*``). The mappings have the contents of ``a`` and
        ``b`` respectively. Both mappings have the structure from ``b``. Any missing
        elements are set to ``Box(None)``.
      """
      out_a = {k: {} for k in b}
      out_b = {k: {} for k in b}
      for k1, v1 in b.items():
          for k2 in v1:
              out_b[k1][k2] = Box(b[k1][k2])
              if k1 in a and k2 in a[k1]:
                  out_a[k1][k2] = Box(a[k1][k2])
              else:
                  out_a[k1][k2] = Box(None)
      return out_a, out_b
  
imports: import collections
  import collections.abc
  from collections.abc import Mapping, MutableMapping
  import functools
  import inspect
  from typing import Any, Callable, Optional, TypeVar
  from haiku._src import base
  import jax
  import jax.numpy
label_desc: Use image node_id 9 for calling a global function with example usage: box_and_fill_missing(a, b) and returns: out_a,  out_b