Yes, PE.

When I saw you being the first to reply I knew something must have gone terribly wrong...

I had trouble with spelling `unsoluble´... somehow while doing a last spelling correction the prefix must have been lost unnoticed.

Luckily enough the rest of the text indicates that there must be a sort of error in this sentence.

Furthermore, Mustafa asked for hints on doing it, so i tried to go not into detail too much and if so to refer to early work.

Please be kind; this compilation of facts was many hours of work.