[NTG-context] Metafun: how to draw a straight line past the end points

Hi, I have a silly question: given two points in the plane z1 = (1cm,3cm) ; z2 = (5cm, 9cm) ; the command draw z1 -- z2 withpen pencircle scaled .5pt ; draws the line segment joining z1 and z2. How could I draw the line segment which goes a little further (say 1cm) than both the end points of this segment (that is in this example, a point before z1 and a point after z2 on the straight line) . Indeed it is possible to write the equation of the line and then compute by hand the coordinates of the new end points, but I think there is a simpler way in Metafun to do this. Thanks for any help: Otared