Add Result and ErrorInfo types; catchResult and Expr.findLhsMatch.
Add rules.removeSimpleMath and rules.removeSubtraction.
Include the substitution in the results of finding LHS matches.
Add rules.simplifyNegations, use it in rules.removeSubtraction.
Rename another rule.
Add Toy.mathText.
Extend Expr.findLhsMatch.
Tune up some simplification rules; use the extended Expr.findLhsMatch.
Add key algebraic simplification rules.