def add_optional(self, dependent, dependency):
        if dependent in self.graph and dependency in self.graph:
            self.add(dependent, dependency)
Compute the step of an excerpt set as a function of the number
dataset/ETHPy150Open kivy/kivy-ios/toolchain.py/Graph.add_optional--crossover with--dataset/ETHPy150Open kwikteam/phy/phy/io/array.py/_excerpt_step