code: def coerce(cls, key, value):
          
          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)
docs: Convert plain dictionary to NestedMutable.