[lkb] Debugging trigger rules

Hi,

I'm working on a grammar for Wambaya, and part of the analysis
relies on a long series of auxiliaries which don't contribute
any relations but do (ultimately) constrain the PNG of the
ARG1 of the verb they combine with.  I'd like to write trigger
rules for these auxiliaries which match on the PNG properties
of the ARG1 of some relation in the list.  Unfortunately, that
doesn't seem to be working.

Here is an example trigger rule:

ngurlu-trigger-rule := generator-rule &
 [ CONTEXT.RELS <! [ ARG1.PNG.PN [ PER excl,
				   NUM du ]] !>,
   FLAGS [ TRIGGER "1du-excl-s-aux" ]].

I would expect this rule not to match when the input MRS has
only the following rels:

< [ _wait+for_v_rel
  LBL h1
  ARG0 e2
  ARG1 x4 [ x PNG.PN.PER: FIRST PNG.PN.NUM: SG ]
  ARG2 x3 ]
[ pronoun_n_rel
  LBL h5
  ARG0 x3 ]
[ exist_q_rel 
  LBL h6
  ARG0 x3
  RSTR h7
  BODY h8 ] >

There's only one rel there with an ARG1, and the PNG features don't
match.  And yet I get edges for the 1du-excl auxiliary associated with
the rule shown when I try to generate from that MRS. 

So what am I missing about trigger rules?

Thanks,
Emily