Things I don't understand about rust part 2

gistfile1.txt

// Why does this work . . .

trait Additive: Add<Self, Self> {}

fn sum<T, U: Zero + Additive>(seq: &[T], f: &fn(&T) -> U) -> U {
    let mut u: U = Zero::zero();
    for seq.each |x| { u += f(x); };
    u
}

// . . . but this doesn't?

fn sum<T, U: Zero + Add<Self, Self>>(seq: &[T], f: &fn(&T) -> U) -> U {
    let mut u: U = Zero::zero();
    for seq.each |x| { u += f(x); };
    u
}

// Callsite:

let mu = do sum(p) |x| { x.mu };