Add rules.copy.
Add Toy.normalReturn, stop setting hasLeftElision in all steps (just rendered ones).
Remove support for automatically simplifying after a step.
Support null rule result in tryRule.
Make applyFacts and similar code operate just on visible parts
of the step.
Make regrouping more powerful; also clean up simplifier code.
Update a rule description.
Comments.
Display unary minus (neg) as "-".
Clean up rules related to negation.