Unification failure in SLASH list

Here’s a unification failure:

The path to that is:
ARGS.FIRST.SYNSEM.LOCAL.CAT.VAL.SUBJ.FIRST.NON-LOCAL.SLASH

What I did here was feed a VP which is output of the complement extraction rule to the subject extraction rule. Of course I expected a unification failure of some kind, so it is not a problem that I have it there. What I want is to learn to analyze them better and faster.

Here’s the subject extraction rule:

basic-extracted-subj-phrase := basic-extracted-arg-phrase & head-compositional &
  [ SYNSEM.LOCAL.CAT.VAL [ SUBJ < >,
                           SPR < >,
                           COMPS < > ],
    HEAD-DTR.SYNSEM [ LOCAL.CAT [ VAL [ SUBJ < gap &
                                             [ LOCAL #local & local &
                                               [ CONT.HOOK.INDEX ref-ind ] ] >,
                                        COMPS olist ],
                                  MC na ],
                      NON-LOCAL.SLASH.LIST < #local > ],
    C-CONT [ RELS <! !>,
             HCONS <! !>,
             ICONS <! !> ] ].

extracted-subj-phrase := basic-extracted-subj-phrase &
  [ SYNSEM.LOCAL.CAT.HEAD verb,
    HEAD-DTR.SYNSEM.LOCAL.CAT.VAL.COMPS < > ].

Is the unification failure pointing me to this line?

HEAD-DTR.SYNSEM [ LOCAL.CAT [ VAL [ SUBJ < gap & ... 

Since the type gap has non-empty SLASH list:

gap := expressed-non-canonical &
  [ LOCAL #local,
    NON-LOCAL [ REL 0-dlist,
		QUE 0-dlist,
		SLASH 1-dlist &
		    [ LIST < #local > ] ] ].

But if I look at the VP which I fed to the rule, its SUBJ’s SLASH seems is 0-1-dlist:

0-1-dlist isn’t the same as null, is it? Where did the rule get the null that it’s complaining about? Does it sound like I am not looking at the right paths after all?

0-1-dlist is a type that is compatible with both <! !> (empty d-list) and <! [ ] !> (dlist with one thing on it). But this particular 0-1-dlist also has LIST null, which means it’s an empty dlist. (I think.)

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Ah yes yes, for some reason I was blind to the LIST null thing! Thanks. I will come back with more questions :slight_smile: :).