diff --git a/src/InvoiceRuleInterface.php b/src/InvoiceRuleInterface.php
index 2de47dd7..46f0d015 100644
--- a/src/InvoiceRuleInterface.php
+++ b/src/InvoiceRuleInterface.php
@@ -11,5 +11,6 @@ interface InvoiceRuleInterface
     public function validateInvoiceItems($criteria, $invoice_items);
 
     // compute
+    // this returns an array of items or empty array
     public function compute($criteria, &$total);
 }