The same `<'a>` name on all of the inputs and outputs tell the borrow checker that these things are related, i.e. they all borrow from the same thing. The borr

The same `<'a>` name on all of the inputs and outputs tell the borrow checker that these things are related, i.e. they all borrow from the same thing. The borrow checker tracks these annotations through all the scopes and function calls back to the "thing" they borrow from, and checks that nothing is borrowed for too long. I think this particular case is trivial enough that the annotation could be omitted: fn add(env: Env, args: &[Term]) -> Result<Term, Error> { but in more complex cases where data from some arguments is returned, and from some it is not, it's useful to annotate which inputs/outputs are related and which aren't, so that the unrelated arguments can be temporary values without tainting everything else as temporary.