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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 clean_prefix(self):
 """ Validates the prefix """

        if self.instance.fixed:
            return self.instance.prefix

        prefix = self.cleaned_data['prefix']
        if not namespace.is_ncname(prefix):
            raise forms.ValidationError("This is an invalid prefix")

        return prefix