How can i find the definition of ?android:attr/dividerVertical?

I want to see the definition of divider vertical, I'm not sure exactly what the meaning of an attribute is but when click through to the definition of the attribute in intellij I'm taken to attrs.xml and shown the following, which does not help.

<!-- Drawable to use for generic vertical dividers. --> 
<attr name="dividerVertical" format="reference" />

My specific problem is I'm trying to achieve a list with an inset list divider with the dividerVertical style. In order to do this I have defined my own inset shape.

<inset xmlns:android="http://schemas.android.com/apk/res/android"
   android:insetLeft="30dip"
   android:insetRight="30dip">
<shape xmlns:android="http://schemas.android.com/apk/res/android"
       android:shape="rectangle">
    <solid android:color="?android:attr/dividerVertical"/>
</shape>
</inset>

This does not work as android will not accept the "?android:attr/dividerVertical" as the colour. It would not work anyway as divider vertical has an alpha component, so what I need to know is what colour and opacity is divider vertical?? But ideally Id like to know how I can check the source of any resource components in android so that I never encounter this problem again.

Thanks

Piers