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

In particular, thanks to (in alphabetical order):

Brian Bartholomew 
David Boddie 
Jeffrey Boser 
Simon Budig 
Yves Ceccone 
Mauro Colorio 
Aaron Digulla 
Michael Dunstan 
Christof Ecker 
Thomas Gellekum 
Martin Glaser 
Philipp Matthias Hahn 
Konrad Hinsen 
Gregor Hoffleit 
Eric Jacobs 
Tamito KAJIYAMA 
Panos Katsaloulis 
Andrey Kiselev 
Steinar Knutsen 
Olof S Kylander  
Tessa Lau 
Frederic Lepied 
Michael Loßin 
Esteban Manchado Velázquez 
Eric Marston 
Han-Wen Nienhuys 
Antoon Pardon 
Bernhard Reiter 
Christian Schwarz 
John Shumway 
Frédéric Toussaint