> a guess, but i'd wager "rare" here meant the code should in the main prefer immutable data structures (etc), not that the invocations per unit time of a particular method should be low.

Yeah I want to say that for us developers, "rare" is not something that we can code, without some serious sophistication.

More likely we code "if" and it is either "yes" or no".

To capture "rare", we need some serious tools - statistics, analytics etc etc. That is way beyond the regular developer toolset.