Reason for using generics in lifelines isn't clear

[mdinger]

In Lifetimes/Functions, the example:

struct Triplet { one: int, two: int, three: int }
impl Triplet {
    // Fails
    fn mut_one(&mut self) -> &mut int { &mut self.one }
    // Fails
    fn mut_two<'s, 'f>(&'s mut self) -> &'f mut int { &mut self.two }
    // Works
    fn mut_three<'s>(&'s mut self) -> &'s mut int { &mut self.three }
}

You switch fromfn mut(...) to fn mut<...>(...) without explanation. Triplet isn't generic so it isn't clear why the lifelines should need to be generic. Removing <...> definitely fails but it's not clear why.