WW: Interlineal sometimes hangs

Cathy Cantwell reports that Interlineal conversion, with
the matrix option enabled, often/sometimes/always
hangs (evidently in an infinite loop) on the penultimate
tsheg bar of a line. She sent a sample file to illustrate
this.

I was able to duplicate this once, but not again. Hitting
Pause sucessfully exited the loop.

I duplicated this loading from Startup (which is the usual
source of nondeterministic bugs). I'm not sure that is
necessary but probably so.