dWz, dWr, dWh = [np.zeros_like(w) for w in (self.wz, self.wr, self.wh)]
dbz, dbr, dbh = [np.zeros_like(b) for b in (self.bz, self.br, self.bh)]
dWhy = np.zeros_like(self.why)
dby = np.zeros_like(self.by)