assurer consistance entre manual et code
(ex: interface intersect)

tracker les vestiges de l'ancienne representation des points (pointeurs
vers cercles)

utiliser partout ou c'est possible le constructeur de arc_point(Point_2)

How can we avoid to call global functions of CK in internal code? 
These calls look rather dirty to me. 
I (MT) tried to remove them but could not: if these calls are replaced 
by calls of internal functions, then CK::Polynomials must be accessed,
but they are undefined in Lazy_kernel...


enlever messages de debug
regarder traits-tracer (seg fault)

construct intersection for *arcs* should use solve or solve in range

add "using..." in all functors that derive from another