fn var(&self) -> f32 {
        let mut xn = 0f32;
        let mut n = 0f32;
        let mut m2n: f32 = 0f32;

        for x in self.iter() {
            n += 1f32;
            let diff_1 = x - xn;
            xn += diff_1 / n;
            m2n += diff_1 * (x - xn);
        }
        assert_ne!(n, 1f32);
        m2n / (n - 1f32)
    }
Variance

Uses welfords online algorithm for numerically stable computation.

# Examples
```
#[macro_use]
extern crate peroxide;
use peroxide::fuga::*;

fn main() {
let a = c!(1,2,3,4,5);
assert_eq!(a.var(), 2.5);
}
```
src/statistics/stat.rs