Pull Iterators

Two warehouses each produce a stream of orders sorted by ID, and a report needs one stream sorted by ID. How can a single loop take the smaller of two values when each value is inside a different iterator?

A range loop cannot express this without first copying a stream into a slice. The iterator controls the loop: it runs, and it calls the loop body whenever it has a value. A loop body over the first stream can start a range over the second stream, but it must end that inner loop before the first stream can send its next value