Compact concatenation in an if block.
Extract parse forest concatenation.
Concatenation of null parses concatenates the parses.
Compact null parse prefixes of concatenations.

Note that this does not pass the tests because cons is not associative.
Merge branch 'master' into compact-concatenation-of-null-parses

Conflicts:
	Hammer/HMRConcatenation.m
More flatter.

This feels pretty awful.
Test more derivatives.
Merge pull request #34 from robrix/compact-concatenation-of-null-parses

Compact concatenation of null parses