Unit Testing - What not to test

Question

I have gone through a number posts on stackoverflow and numerous articles about Unit Tests. I am just trying to figure out that what I have understood is right.

Answer 1

I'm not unit-testing anything, because instead I want the integration and/or system tests to:

• Cover the code
• Be automated
• Be used as, and to be as useful as, unit tests

For details, see Should one test internal implementation, or only test public behaviour?

Reversing your question, I suggest that the only code which you should unit test is code:

a) Which isn't going to be tested at all by integration and/or system testing (and if not, why not?)

b) Or which should, for whatever reason, be tested before integration/system testing (perhaps because of how which your project's development is partitioned to several developers in parallel)