Mathema Solfege. I bought the single back in 2010, but as it turns out, there are several different versions of this single.
In the UK, this single without a picture sleeve was released, featuring an English version called 'A music lesson'. The original version appears on the B-side. There is also a German single, which features the German version 'Freunde der Nacht' and 'A music lesson'. It commands prices that are quite impressive, starting at 15 euros.
My collection: 7" single no. 5688
Found: Discogs.com, received December 31, 2014
Cost: 2 euro
Tracks: 'A music lesson' / 'Mathema solfege'