What, if anything, can be done in linear time?

The answer to the title question seems to be “Not much.” Even sorting $n$ items takes $n \times \log ( n )$ swaps. But, in fact, quite a bit can be done in linear time. We start with an illustration of linear-time techniques. Then we sketch the linear-time decision algorithm for propositional primal infon logic. Finally we mention useful extensions of that logic decidable in deterministic or probabilistic linear time.