In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.
You'll want to check the chapters "Self-referenced variables are removed" and "Resolving a problem with a mutable variable in a security expression".
Afterwards you can automatically migrate version 3 to version 4: