wizard_answer.md

In this particular case you don't even need a custom accessor. You can create extra constraints on your base type constraint with a where clause:

class Wizzard {
    has Int $.mana is rw where * > 0;
}

But if you did want to create a custom accessor you simply have to define a method with the same name and add the is rw trait.

method mana is rw {
   say "running out of mana" if $!mana < 20;
   $!mana;
}

You will still be able to use equals assignment syntax like $wizard.mana = 50 but you will not be able to capture the value before it is assigned. If you want something even more fine grained explain Proxy