%% syntaxe : alpha ell Epoint --> le point M de elle tel que arg (IM)
%% = alpha (ou I est le centre de l'ellipse ell)
/Epoint {
8 dict begin
   /ell exch def
   /theta exch def
   ell aload pop
   /alpha exch def
   /theta theta alpha sub store
   /b exch def
   /a exch def
   pop pop 
   theta cos 0 eq
      {
         theta ell epoint
      }
      {
         theta tan a mul b div theta cos signe atan /t exch def
         theta cos 0 lt
            {
                /t t neg store 
            }
         if
         t ell epoint
      }
   ifelse
end
} def