fix parsing of quoted lists, etc.

The commit history is a bit muddled, so I'm not certain of the
motivation for changing it to its current state.

The below change should fix the problem.

I'm not clear what a symbolExpr is though. I think it represents an
identifier. That's why below I converted it to a quotedExpr (which, I
think, is a Scheme symbol).

We should probably add a test case for '(1 a "a" (2 b "b" (3) f)) as
it catches many of the problems.