Concatenate concatenation.
Merge branch 'master' into flatten-concatenation
Repetition is intended to be flat.
Extract parse forest concatenation.

Conflicts:
	Hammer/HMRConcatenation.m
Use reducers to concatenate parse forests.
Not car/cdr.
Reduce nontermination to preserve structure.

Recursion on its own is going to be a bit of a pain to flatten.
Merge pull request #33 from robrix/flatten-concatenation

Flatten concatenation