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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 meta_set(self, key, metafield, value):
        ''' Set the meta field for a key to a new value. This triggers the
            on-change handler for existing keys. '''
        self._meta.setdefault(key, {})[metafield] = value
        if key in self:
            self[key] = self[key]