metafun question getting XY

[Floris van Manen <vm@klankschap.nl>]

if i define a path

q := (w0,w0)--(w0,ww0)--(w2,ww1)--(w3,ww2)--(w3,w0)--cycle ;

and then pass this path as a parameter to a function

test( q ) ;


then within that function i can step through the 'list of points'.

def test( expr a  ) =
 for i = 1 upto (length(a)-2):
  dotlabel.bot ("" , (point i of a)  );
 endfor ;
enddef;


but how could i extract from each point the X and the Y value to
construct the lines like this.

 for i = 1 upto (length(a)-2):
  z1 = (point i of a) ;
  draw (0,y1)--(100,y1) ;
 endfor;



thanks!
% floris