Function split 

pub fn split<B: Send>(
    out: &mut [B],
    unit: usize,
    write: &(dyn Fn(usize, &mut [B]) -> bool + Sync),
) -> bool

Runs write(start, items) over disjoint pieces of out, using ranges in units of unit items, and gives whether every call said true. An empty output calls write once. Panics if unit is zero.