Basic signup for events

app/ValueObjects/Amount.php

@@ -45,6 +45,11 @@ class Amount {
         $this->amount -= $amount->getAmount();
     }
 
+    public function multiply(float $factor) : Amount {
+        $this->amount *= $factor;
+        return $this;
+    }
+
     public function getFormattedAmount() : string {
         $value = number_format( round( $this->amount, 2 ), 2, ',', '.' );
         return $value