Many thanks to all who have commented on Sketch, sent suggestions and
bug reports and helped in testing. 

In particular, thanks to:

Olof S Kylander  
Esteban Manchado Velázquez 
Panos Katsaloulis 
Martin Glaser 
Gregor Hoffleit 
Konrad Hinsen 
Christian Schwarz 
Frederic Lepied 
Aaron Digulla 
Brian Bartholomew 
Steinar Knutsen 
Michael Dunstan