Using `fold` vs `sum` in *Higher Order Functions*

[alexfertel]

I noticed that the example comparing imperative vs functional approaches uses fold vs sum.

One can argue that fold is classically thought of as functional, but I think that the objective of the example is the functional flavor, thus sum would be more appropriate.

    let sum_of_squared_odd_numbers: u32 =
        (0..).map(|n| n * n)                             // All natural numbers squared
             .take_while(|&n_squared| n_squared < upper) // Below upper limit
             .filter(|&n_squared| is_odd(n_squared))     // That are odd
             .fold(0, |acc, n_squared| acc + n_squared); // Sum them

vs

    let sum_of_squared_odd_numbers: u32 =
        (0..).map(|n| n * n)                             // All natural numbers squared
             .take_while(|&n_squared| n_squared < upper) // Below upper limit
             .filter(|&n_squared| is_odd(n_squared))     // That are odd
             .sum();                                     // Sum them