impl Exp {
    /// Construct a new `Exp` with the given shape parameter
    /// `lambda`. Panics if `lambda <= 0`.
    #[inline]
    pub fn new(lambda: f64) -> Exp {
        assert!(lambda > 0.0, "Exp::new called with `lambda` <= 0");
        Exp { lambda_inverse: 1.0 / lambda }
    }
}

impl<X: SampleUniform + PartialOrd> WeightedIndex<X> {
    /// Creates a new a `WeightedIndex` [`Distribution`] ...
    pub fn new<I>(weights: I) -> Result<WeightedIndex<X>, WeightedError> where ... {}
}

/// Error type returned from `WeightedIndex::new`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum WeightedError {
    /// The provided iterator contained no items.
    NoItem,

    /// A weight lower than zero was used.
    NegativeWeight,

    /// All items in the provided iterator had a weight of zero.
    AllWeightsZero,
}