RFC 2738 (rfc2738) - Page 3 of 5

Corrections to "A Syntax for Describing Media Feature Sets"

RFC 2738        Corrections to Syntax for Media Features   December 1999


2. Correction to feature expression syntax

   In section 4.1, RFC 2533 defines the syntax for a "set" expression as
   follows:

       set        =  attr "=" "[" setentry *( "," setentry ) "]"
       setentry   =  value "/" range

   The production for 'setentry' should read:

       setentry   =  value / range

   That is: the '/' character is not a character literal, but separates
   two alternative forms for 'setentry'.  This corrected syntax allows
   the set expression examples given in section 4.2.5 of RFC 2533, such
   as:

       ( width=[3,4,6..17/2] )

3. Correction to feature set matching reduction rules

   In section 5.8.2, "Rules for simplifying unordered values", RFC 2533
   lists the following rewriting rules for simplifying feature tag
   comparisons with unordered values:

      (LE f a)  (LE f b)      -->  (LE f a),   a=b
                                   FALSE,      otherwise
      (LE f a)  (GE f b)      -->  FALSE,      a!=b
      (LE f a)  (NL f b)      -->  (LE f a)    a!=b
                                   FALSE,      otherwise
      (LE f a)  (NG f b)      -->  (LE f a),   a!=b
                                   FALSE,      otherwise

   The second of these rules is incomplete, and should read:

      (LE f a)  (GE f b)      -->  (LE f a),   a=b
                                   FALSE,      otherwise

        NOTE:  implementation experience with these rules has
        suggested a revised feaure set matching algorithm with a
        more useful set of simplification rules.  Apart from the
        change noted above, the algorithm given in RFC 2533 has
        been implemented and shown to work as intended, but the
        results generated are not always in the most convenient
        form.  It is planned to test and publish a revised
        algorithm at a future date.





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