/// Combine all an iterator's elements into one element by using [`Extend`].

///

/// [`IntoIterator`]-enabled version of [`Itertools::concat`](crate::Itertools::concat).

///

/// This combinator will extend the first item with each of the rest of the

/// items of the iterator. If the iterator is empty, the default value of

/// `I::Item` is returned.

///

/// ```rust

/// use itertools::concat;

///

/// let input = vec![vec![1], vec![2, 3], vec![4, 5, 6]];

/// assert_eq!(concat(input), vec![1, 2, 3, 4, 5, 6]);

/// ```

pub fn concat<I>(iterable: I) -> I::Item

where

    I: IntoIterator,

    I::Item: Extend<<<I as IntoIterator>::Item as IntoIterator>::Item> + IntoIterator + Default,

{

    iterable

        .into_iter()

        .reduce(|mut a, b| {

            a.extend(b);

            a

        })

        .unwrap_or_default()

}