Thanks to all of them.
Greg Reese and M.Angeles Lopez Malo, who told me how to represent the data properly. Sven Neumand and the plug-in skeleton he sent to me. Johnny Tevessen, for his example on automake/autoconf. Mario De Weerd, who updated GFourier to Gimp 1.2. Lots of GIMP and plug-ins developers, whose work I searched for help. Several other people, who sent ideas and bug reports.