sub field_promotion (Soldier $x is rw)
 {
    $x = Private.new;  # Private "isa" Soldier, allowed.
 }

my General $y .= new;
field_promotion ($y);  # General "isa" Soldier, allowed.
$y.order_attack();

##</code><code>##

void field_promotion (Soldier& x)
 {
    x = Private();  // value assignment slices object (if allowed at all)
 }

##</code><code>##

void field_promotion (Soldier* &x)
 {
    x= new Private();  // allowed, and updates caller's pointer variable
 }
void caller()
 {
 General* y= new General();
 field_promotion (y);  // compile-time error
 }

##</code><code>##

our Soldier $x;

multi sub trythisone (General $y)
 {
 # $y is a read-only alias of the caller's variable, global $x in this case.
 # This function chosen based on actual type of $x at the time the call was made.
 $x = Private.new;
 # what just happened to $y?
 $y.order_attack();
 }

$x = General.new;
trythisone ($x);