for my project I need to build a tree structure. I am looking for a way to grow it at the leaves. I have simplified my failed attempt by using a listy structure:

```
my $root = a => (b => (c=> Nil));
my $here := $root;
while $here.value ~~ Pair {
  $here := $here.value;
}
$here = d => Nil;

```

which does not work, because I cannot change Nil, of course.
 Cannot assign to an immutable value
How can I make this work?

Thanks,
Theo van den Heuvel