Return the edges available from fromVol.

def getEdges(self, fromVol):
        """ Return the edges available from fromVol. """
        if fromVol is None:
            for toVol in self.paths:
                yield Store.Diff(self, toVol, fromVol, toVol.size)
            return

        if fromVol not in self.paths:
            return

        fromBVol = self.butterVolumes[fromVol.uuid]
        parentUUID = fromBVol.parent_uuid
        butterDir = os.path.dirname(fromBVol.fullPath)

        vols = [vol for vol in self.butterVolumes.values()
                if vol.parent_uuid == parentUUID or
                os.path.dirname(vol.fullPath) == butterDir
                ]

        changeRate = self._calcChangeRate(vols)

        for toBVol in vols:
            if toBVol == fromBVol:
                continue

            # This gives a conservative estimate of the size of the diff

            estimatedSize = self._estimateSize(toBVol, fromBVol, changeRate)

            toVol = self._btrfsVol2StoreVol(toBVol)

            yield Store.Diff(self, toVol, fromVol, estimatedSize, sizeIsEstimated=True)