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

[hiSandog]

A useful next step would be a minimal fixture for Nested default does not accept muscle children in PyMJCF that includes one failing input and one nearby control case. That would make it clear whether the fix addresses the reported edge case without widening behavior unexpectedly.