The sentence "The names of lifetime parameters are usually all lowercase, and like generic types, their names are usually very short. 'a is the name most people use as a default." is made confusing, in my opinion, by the addition of the last three superfluous words. The word "default" carries with it an implication of a value the software supplies if you don't. That's not at all what is being talked about there. It's simply the choice of the name for a lifetime. I think this would be improved by eliminating the last three words: "The names of lifetime parameters are usually all lowercase, and like generic types, their names are usually very short. 'a is the name most people use."