proof of concept for "is tracked" variable trait

gistfile1.txt

class History is Proxy {
    has @.history;
    method history() { @.history.skip.map: *<> }
    method push(\value --> Nil) { @!history.push(value) }
    method reset(--> Nil) { @!history = () }
}

sub historized-container() is raw {
    my $value;
    my $historized-container := History.new(
      FETCH => -> $ { $value },
      STORE => -> $, \new-value {
          $historized-container.VAR.push($value);
          $value = new-value
      }
    )
}

multi sub trait_mod:<is>(Variable:D $variable, Mu :$tracked! --> Nil) is export {
    (my $name := $variable.var.VAR.name)
      ?? (OUTER::{$name}:exists)
        ?? (OUTER::{$name} := historized-container)
        !! die $name.substr(1)
          ?? "Could not find container by name '$name'"
          !! "Can not track containers without name"
      !! die "Can not track unnamed containers";
}

my $alphabet is tracked = 42;
$alphabet = 666;
dd $alphabet.VAR.history;