def init(self, filename):

        (File(filename, "r") + Line()).register(self)
This method is overwritten because we need to modify kdiag by substituting z for x. We do this by calling the parent expression method to extract variables from expressions, then subsitute the z variables that are present with x.
dataset/ETHPy150Open circuits/circuits/examples/99bottles.py/Tail.init--crossover with--dataset/ETHPy150Open SheffieldML/GPy/GPy/kern/src/symbolic.py/Symbolic._set_expressions