[DKF]: By parallel with [?set], and only relevant from Tcl 8.5 onwards:
======
proc ?incr {v_name {increment 1}} {
    upvar $v_name v
    if {![info exists v]} {
        return -code error "expected $v_name to exist!"
    }
    if {[array exists v]} {
        return -code error "can't increment \"$v_name\": variable is array"
    }
    incr v $increment
}
======





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