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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def coerce(cls, key, value):
 """ Convert plain list to MutableList. """

        if not isinstance(value, MutableList):
            if isinstance(value, list):
                return MutableList(value)
            # this call will raise ValueError
            return Mutable.coerce(key, value)
        else:
            return value