tracks/csharp/exercises/pascals-triangle/Example.cs in trackler-2.2.1.41 vs tracks/csharp/exercises/pascals-triangle/Example.cs in trackler-2.2.1.42

@@ -1,16 +1,25 @@
 using System.Collections.Generic;
+using System;
 
 public static class PascalsTriangle
 {
     public static IEnumerable<IEnumerable<int>> Calculate(int rows)
     {
+        if (rows < 0)
+            throw new ArgumentOutOfRangeException();
+
+        return IterateRows(rows);
+    }
+    
+    private static IEnumerable<IEnumerable<int>> IterateRows(int rows)
+    {
         for (var i = 1; i <= rows; i++)
         {
             yield return Row(i);
         }
     }
-    
+
     private static IEnumerable<int> Row(int row)
     {
         yield return 1;
         var column = 1;
 
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