proc ?append {v_name args} {
   upvar $v_name v
   if {![info exists v]} {
     return -code error "expected $v_name to exist!"
   }
   foreach val $args {
     append v $val
   }
 } ;# GPS

[DKF]: I've updated the code above to match the calling convention for [append]
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See also [?set], [?lappend]
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[Category Command]