code: def coerce(cls, key, value):
          "Convert plain dictionaries to MutableDict."
  
          if not isinstance(value, MutableDict):
              if isinstance(value, dict):
                  return MutableDict(value)
  
              # this call will raise ValueError
              return Mutable.coerce(key, value)
          else:
              return valuf coerce(cls, key, value):
          "Convert plain dictionaries to MutableDict."
  
          if not isinstance(value, MutableDict):
              if isinstance(value, dict):
                  return MutableDict(value)
  
              # this call will raise ValueError
              return Mutable.coerce(key, value)
          else:
              return value
docs: Convert plain dictionaries to MutableDict.