Link to Youtube instead of screencast.com for video.
Fix bug in Expr.concat.
Test bug fix.
Get current set of division facts working.  Rethrow proof errors in getResult.
Restore highlighting of steps that reference the current step.
Also fix another bug.
Remove unused removeStepAndDeps.
Move highlight removal code into ProofControl.removeStep, renamed as public.
Fix ProofControl.removeStep and bulk removal, preventing issues with
removing steps still needed in some way.
Make ProofControl.removestep self-contained by clearing any subproof there.
Rename and slightly modify ProofControl.removestep to be removeLastStep,
reflecting what it can do properly.