The case for providing multi-arg'd coroutines is:

   * coroutines should be able to emulate any command, not just any single-arg'd command [[generality]]
   * to implement single-arg's coroutines in multi-arg'd coroutines is simple.  The converse is inefficient and difficult.  [[increase in expressive power]]
   * there is no sound reason that the invocation of a coroutine should not resemble that of any other command [[principle of minimal surprise]]

CASE 1: (counterfactual)

Assume a [Coroutine] which generates a command taking multiple args, to implement [coroutine] as we have it implemented:

======
proc coroutine {name command args} {
    set ns [namespace qualifiers $name]
    if {$ns eq ""} {
        set ns [uplevel 1 {namespace current}]
    }
    set name [namespace tail $name]

    Coroutine ${ns}::$name $command {*}$args
}
======
To provide precisely the same functionality as [yield] it is necessary to strip off a single layer of [list]:
======
proc ::yield2 {value} {
    return [lindex [::yield $value] 0]
}
======
No other changes are necessary.  More likely, one would define ::yield like that, and create a new ::yield-variant which returned the whole invocation arg list.

CASE 2:
[[Coroutine]] in [[[coroutine]]] - implementing multi-arg'd coroutines over singe-arg'd [coroutine]

======
proc Coroutine {name command args} {
    set ns [namespace qualifiers $name]
    if {$ns eq ""} {
        set ns [uplevel 1 {namespace current}]
    }
    set name [namespace tail $name]

    set coco [::coroutine ${ns}::_C$name $command {*}args]
    trace add command ${ns}::_C$name delete "rename ${ns}::name {}"
    proc ${ns}::$name {args} {
        set name [lindex [info level 0] 0]
        set ns [namespace qualifiers $name]
        if {$ns eq ""} {
            set ns [uplevel 1 {namespace current}]
        }
        set name [namespace tail $name]
        
        ${ns}::_C$name $args
    }
}
======

Counterfactual side-benefits:  [::yield] stays exactly as it is.

Yield to populate args becomes simpler.
======
proc yieldv {value args} {
    uplevel 1 [::yield $value] {*}$args
}
======

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