fn execute_into<I, OP, FromA, FromB>(
    a: &mut FromA,
    b: &mut FromB,
    pi: I,
    op: OP,
)where
    I: ParallelIterator,
    OP: UnzipOp<I::Item>,
    FromA: Send + ParallelExtend<OP::Left>,
    FromB: Send + ParallelExtend<OP::Right>,Expand description
Runs an unzip-like operation into ParallelExtend collections.