appending to a file with open

[tob at b-e-s-t.com]

I have a simple prc that writes the total records to a file.

cat prc.left
::end:
@wgt:::
::az(50,*)="/u/tob/ckinvct":
::aa=open(az,"rwt"):
::az=@qu<@ts:
::ab=writeline(aa,az):
::end:

It works as expected BUT it does not append to the file but creates a new one on every run.  I would like it to append to the file.
Thanks
centos 5 / fp 5