def push(self, value=None):
        if not isinstance(value, Bottle):
            value = Bottle()
        self.append(value)
        return value
Returns the expression as a tuple wrt the following
dataset/ETHPy150Open adampresley/bottlepy-bootstrap/app/bottle.py/AppStack.push--crossover with--dataset/ETHPy150Open sympy/sympy/sympy/vector/basisdependent.py/BasisDependent.as_numer_denom