Lookup CONTEXTJ test

[Simon Josefsson]

Hi,

I need help with interpretation regarding section 5.4 which says:

   Putative U-labels with any of the following characteristics MUST be
   rejected prior to DNS lookup:
...
   o  Labels containing code points that are identified in the Tables
      document as "CONTEXTJ", i.e., requiring exceptional contextual
      rule processing on lookup, but that do not conform to those rules.

I have trouble understand the bullet text.  To me, it seems as if the
first part of the sentence, namely:

    Labels containing code points that are identified in the Tables
    document as "CONTEXTJ"

says one thing but the rest of the sentence, namely:

      requiring exceptional contextual rule processing on lookup, but
      that do not conform to those rules.

says a different thing.

What is not clear to me is whether the test on a particular label is
intended to fail if and only if:

1) the label has any code point with the CONTEXTJ property.

2) the label has any code point with the CONTEXTJ property AND the rule
   fails.

Interpretation 2) makes the most sense to me, but the normative part of
the sentence suggests otherwise so I am looking for clarification.

The text goes on and says:

      Note that this implies that a rule must be defined, not null: a
      character that requires a contextual rule but for which the rule
      is null is treated in this step as having failed to conform to the
      rule.

What is a "null rule"?  I cannot find any definition.

/Simon