Nested default does not accept muscle children in PyMJCF

[hudsonburke]

Hi, I am having an issue with PyMJCF handling nested default classes containing muscle elements.

Environment

• dm_control==1.0.41
• mujoco==3.8.1

Minimal repro

  from dm_control import mjcf

  xml = """<mujoco model="repro">
    <default>
      <default class="forearm_muscle">
        <muscle ctrllimited="true" ctrlrange="-1 1" />
        <tendon width="0.002" />
      </default>
    </default>
  </mujoco>
  """

  mjcf.from_xml_string(xml)

Observed behavior

This fails with:

  KeyError: "Line 4: error while parsing element <muscle>: 'muscle'"

Manual construction also fails

  from dm_control import mjcf

  root = mjcf.RootElement(model="repro")
  d = root.default.add("default", dclass="forearm_muscle")
  d.add("muscle", ctrllimited="true", ctrlrange="-1 1")

This raises:

  AttributeError: <muscle> is not a valid child of <default>

Additional observation

  from dm_control import mjcf

  root = mjcf.RootElement(model="debug")
  outer = root.default
  inner = outer.add("default", dclass="forearm_muscle")

  print("muscle" in outer.spec.children)  # True
  print("muscle" in inner.spec.children)  # False

So it looks like the top-level default accepts muscle, but nested default does not.

Expected behavior

I would expect nested default classes to accept <muscle .../> children as well.

This came up while parsing a musculoskeletal MJCF model that uses blocks like:

  <default class="forearm_muscle">
    <muscle ctrllimited="true" ctrlrange="-1 1" />
    <tendon width="0.002" />
  </default>

I am fairly new to using this parser, so please let me know if there is something I am missing