Fix error in previous change.
Fix Expr.isReal.
Display variables better in a couple of cases.
Annotate all rendered steps with type information.
Annotate all facts with type information.
Add Expr.hasType, export Toy.TypeConstant.
Annotate all goals of Fact objects with type information for future reference.
Make checking of facts for offerability smarter, using type information in the facts
and the target expressions.
Improve a rule comment.
Fix deficiency in presentation of equational facts in menus.
Add another sample of instMultiVars.
Initialize the sourceStep property of Exprs, add comments about these properties.
Semi-automatically flag hypotheses with source steps.