Computed columns cannot be added to Document library

Using a feature I have added three site columns, one of which is computed. The two not-computed columns can be found under "Site Columns" in the Site Administrator window:

But what about the computed site column? The computed column is available under the site

oWeb.AvailableFields()

but doesn't show up above - neither it shows up under the Edit View (All Documents):

So - how do I add a computed field - preferably with a picture - in a view?

Further - very often Site Columns are not updated and displayed - despite an IISRESET - so they are simply not present :-(