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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 _gen_code(self):
 """Generate code for the list of expressions provided using the common sub-expression eliminator to separate out portions that are computed multiple times."""

        # This is the dictionary that stores all the generated code.

        self.code = {}
        def match_key(expr):
            if type(expr) is dict:
                code = {}
                for key in expr.keys():
                    code[key] = match_key(expr[key])
            else:
                arg_list = [e for e in expr.atoms() if e.is_Symbol]
                code = self._expr2code(arg_list, expr)
            return code

        self.code = match_key(self.expressions)