Source PostScript (xdpoint.pps)

%% x A B xdpoint : le point de la droite (AB) d'abscisse x
/xdpoint {
5 dict begin
   /pt2 defpoint
   /pt1 defpoint
   /x exch def
   /a pt1 pt2 coeffdir def
   /b pt1 pt2 ordorig def
   x dup a mul b add
end   
} def