case out <- v:
				}
			}
		}
	}

	selectFrom := func(idx int) error {
		select {
		case <-ctxDone:
			return ctx.Err()
		case entry, ok := <-in[idx]:
			if !ok {
				top[idx] = nil
				nDone++
			} else {
				top[idx] = &entry
			}
		}
		return nil
	}
	// Populate all...
	for i := range in {
		if err := selectFrom(i); err != nil {
			return err
		}
	}