for executing the loop body once.
After each successive loop iteration, <code>yield</code> returns true
and may be called again to continue the loop.
As long as the loop body does not terminate, the "range" clause will continue
to generate iteration values this way for each <code>yield</code> call until
<code>f</code> returns.
If the loop body terminates (such as by a <code>break</code> statement),