From a lady on the TT forum:
[quote]You've warmed up on the small ring.
You change to big ring for the intervals and SNAP goes the front changer cable and dumps you back on the small ring.
Hunt down a suitably-sized pebble from the verge;
Jam it into the parallelogram-inner-shape of the front changer to keep it out on the big ring;
Wrap and knot it in place with the broken front cable.
Hey presto. Continue with intervals as planned. Takes more than a broken cable to stop me...
At end of intervals, remove pebble so you can have a nice spin home on the little ring.