Import reducers in the pch.
Reductions reduce to parse forests via Reducers.
Subclassing hook, because why not.
Treat empty arrays as ignorable.
Tidy repetition derivatives.
Parse repetitions more simply.
Assign the first approximation to parseForest.

This breaks out of an infinite loop which occurred in the parsing of
certain recursive repetitions.
Test recursive repetitions.
Merge pull request #22 from robrix/flattened-concatenation-parse-trees

Flattened concatenation parse trees