Evaluates extensions in the registry.  If some extension handles the
        format string, it returns a string.  Otherwise, returns ``None``.

def eval_extensions(self, value, name, option, format):
        """Evaluates extensions in the registry.  If some extension handles the
        format string, it returns a string.  Otherwise, returns ``None``.
        """
        try:
            exts = self._extensions[name]
        except KeyError:
            raise ValueError('no suitable extension: %s' % name)
        for ext in exts:
            rv = ext(self, value, name, option, format)
            if rv is not None:
                return rv