OBEY THE LAW OF CAR.

car is only defined for non-empty lists
Fix undefined variable in parser.

Broke in 2dffa5779d8da81f3870121d9723516466520375
car for nested lists.
Run tests in order.

I don't suck, honest, but the tests build up from simpler
to more complex so it's useful to have them ordered so the
more fundamental breakages can be fixed first.
Recursively parse expressions.