def difference(self,a,b): return [so2.diff(a[0],b[0])]
def difference(self, a, b): return [so2.diff(a[0], b[0])]
def distance(self,a,b): return abs(so2.diff(a[0],b[0]))
def distance(self, a, b): return abs(so2.diff(a[0], b[0]))