Variables declared within the main body of a [coroutine] persist for the life of the coroutine.  This much is obvious.  What may be less obvious is that you can access those variables from commands called from the coro, using [[[upvar] #1]], so here's a little wrapper to facilitate that:

    proc corovars {args} {
        foreach n $args {lappend v $n $n}
        uplevel 1 [list upvar #1 {*}$v]
    }

So, coro-persistent variables may be declared thus within procs:

    corovars x y z

[MS] a variant that builds the list in a more direct manner is
    
    proc corovars {args} {
        set cmd [list upvar #1]
        foreach n $args {lappend cmd $n $n}
        uplevel 1 $cmd
    }
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