Clean up some styling.
Clean up step event handling code.
Tweak rendering of the step deleter button.  Chrome rendering had
a glitch with the old "height" property too.
More cleanup of step event handling code.
Tidy up CSS just a bit.
Clean up some event handling for steps; try out more delegation.
Use on-page error reporting more rather than alerts.
Add more facts.
Add a fact.
Fix tautology tests to use ==.
Use a, b, c in facts rather than x, y, z.  As an aid to this,
in getResult auto-convert variable names to match the desired statement.
Use a vs. x in a couple more facts; remove redundant ones.
Add support for /main in Call.replaceAt (bug fix).
Complete support for /right, /left, /binop, /main in paths.
Correct handling of /main in paths; handle /left, /right, etc
directly in paths.
Add a couple of facts.
Add Expr.prettyPathTo and tests.
When filling in rule args from UI selections, use pretty paths
for human scrutability.  For facts show the toString form
rather than a dump, also for scrutability.
More negation and subtraction facts.
Clerically tidy some proofs of facts.