2839
2840
2841 - no need to check inputs in trace

Just make sure we specify the top-level eval, so we don't
pass on intermediate nils, etc.
2842 - figured out how to check 'not nil'
2843 - interactive-mode bug

How to write tests for this?
2844 - no more read() in tests
2845 - more robust eval trace

Now most tests will continue to pass when we redefine +, etc.

But we're starting to see Dan Oblinger's concern of tracing dominating
logic..
2846