Customizing ProgressBar and SeekBar

Something is missing in the snippet below. In the input field fill in the gap to make correct progress drawable.

<?xml version="1.0" encoding="utf-8"?>
<!--our_progress_drawable.xml-->

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:id="@android:id/background">
        <!-- content -->
    </item>
    <!--  Something is missing here!  -->
    <item android:id="___________________">
        <!-- content -->
    </item>

    <item android:id="@android:id/progress">
        <!-- content -->
    </item>

</layer-list>