Maps showing how FSA have recently been dealing with their eastern or left flank north-east of Al Bab, with simple annotations by myself (in red). West of the thick red vertical line in the upper map shows a solid advance, but with trouble pushing down from the north. The salient in the lower map (lost in the upper map) has been marked “A bridge too far”, just to illustrate it, not because I know what their mission was there – I don’t; but they may have overextended, or had a specific target before withdrawing.
Note: the (unmarked) advance south towards Qurt is interesting. If they maintain it, they may cut off the enemy to the west, assuming inertia by Daesh (not likely, maybe).