<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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