def coerce(cls, key, value):
        """Convert plain dictionary to NestedMutable."""
        if value is None:
            return value
        if isinstance(value, cls):
            return value
        if isinstance(value, dict):
            return NestedMutableDict.coerce(key, value)
        if isinstance(value, list):
            return NestedMutableList.coerce(key, value)
        return super(cls).coerce(key, value)