def relpath(self, start=None):
        start = start or self
        rpath = os.path.relpath(self.root, start.dname)
        rpath = os.path.join(rpath, self.fname)
        return rpath
Overloading of the '+' operator. for more control, see self.add
dataset/ETHPy150Open ialbert/pyblue/pyblue/engine.py/File.relpath--crossover with--dataset/ETHPy150Open SheffieldML/GPy/GPy/kern/src/kern.py/Kern.__add__