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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 all_solidity_variables_read(self):
 """ recursive version of solidity_read """

        if self._all_solidity_variables_read is None:
            self._all_solidity_variables_read = self._explore_functions(
                lambda x: x.solidity_variables_read)
        return self._all_solidity_variables_read