cairo pdf

How do I manage to produce a pdf file.

http://cairographics.org/cairo-ocaml/ tells me pdf is supported, and 
there's a simple example of using png.  But I can't figure out what I 
have to do to get a pdf file.

The example as is, does produce a nice triangle.png file, which I can 
view in a browser.  I've got that much working. But it's a pdf file I 
need.

The obvious
   open Cairo.pdf
and later
   Cairo_pdf.surface_write_to_file surface "triangle.png";

doesn't work.

Error: Unbound value Cairo_pdf.surface_write_to_file

Nor does it work if I replace 

   let surface = Cairo.image_surface_create Cairo.FORMAT_ARGB32 ~width 
~height in

by

  let pout =  (open_out_bin "triangle.pdf") in
  let surface = Cairo_pdf.surface_create_for_channel pout (float_of_int 
width) (float_of_int height) in

with at the end

  Cairo.surface_flush surface;
  close_out pout;

I use the following command line for this:

	ocamlopt -I +cairo -I +glMLite cairo.cmxa wander.ml -o wander

It compiles and runs, but creates an empty triangle.pdf file.

What is it I *do* have to do?  It's probably a trivial variation on the 
above, but I can't figure it out.