The following command emits the next Fibonacci number each time it is called.

======
proc f {} {
    yield [set a 0]
    yield [set b 1]
    while 1 {
        yield [set a [expr {$a + $b}]]
        lassign [list $b $a] a b
    }
}
coroutine fib f
======

The call to `[coroutine]` is considered to be fib(0) and returns 0. Add a nullary `[yield]` before the first `yield` to make the first call to `fib` the zeroth call.

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