Function summed_of 

pub fn summed_of<B, T>(
    items: &mut [B],
    unit: usize,
    least: usize,
    sums: &mut [T],
    parts: &mut Vec<T>,
    piece: &(dyn Fn(usize, &mut [B], &mut [T]) + Sync),
)
where
    B: Send,
    T: Copy + Send + Zero,

summed over the units of items, each of unit items, where piece(first, items, sums) gets the items of the piece whose first unit is first.