rfc2533.txt

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   'filter' is the predicate body. It may contain references to the
   formal parameters, and may also contain references to feature tags
   and other values defined in the environment in which the predicate is
   invoked.  References to formal parameters may appear anywhere where a
   reference to a feature tag ('ftag') is permitted by the syntax for '
   filter'.

   The only specific mechanism defined by this memo for introducing a
   named predicate into a feature set definition is the "auxiliary
   predicate" described later.  Specific negotiating protocols or other
   specifications may define other mechanisms.

      NOTE:  There has been some suggestion of creating a registry for
      feature sets as well as individual feature values.  Such a
      registry might be used to introduce named predicates corresponding
      to these feature sets into the environment of a capability
      assertion.  Further discussion of this idea is beyond the scope of
      this memo.



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6.1.2 Invoking named predicates

   Assuming a named predicate has been introduced into the environment
   of some other predicate, it can be invoked by a filter 'ext-pred' of
   the form:

      ext-pred   =  fname *param
      param      =  expr

   The number of parameters must match the definition of the named
   predicate that is invoked.

6.1.3 Auxiliary predicates in a filter

   A auxiliary predicate is attached to a filter definition by the
   following extension to the "filter" syntax:

      filter     =/ "(" filtercomp *( ";" parameter ) ")"
                    "where" 1*( named-pred ) "end"

   The named predicates introduced by "named-pred" are visible from the
   body of the "filtercomp" of the filter to which they are attached,
   but are not visible from each other.  They all have access to the
   same environment as "filter", plus their own formal parameters.
   (Normal scoping rules apply: a formal parameter with the same name as
   a value in the environment of "filter" effectively hides the
   environment value from the body of the predicate to which it
   applies.)

      NOTE:  Recursive predicates are not permitted.  The scoping rules
      should ensure this.

6.1.4 Feature matching with named predicates

   The preceding procedures can be extended to deal with named
   predicates simply by instantiating (i.e. substituting) the predicates
   wherever they are invoked, before performing the conversion to
   disjunctive normal form.  In the absence of recursive predicates,
   this procedure is guaranteed to terminate.

   When substituting the body of a precdicate at its point of
   invocation, instances of formal parameters within the predicate body
   must be replaced by the corresponding actual parameter from the point
   of invocation.







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6.1.5 Example

   This example restates that given in section 4.3 using an auxiliary
   predicate named 'Res':

      (| (& (Pix-x=1024) (Pix-y=768) (Res Res-x Res-y) )
         (& (Pix-x=800)  (Pix-y=600) (Res Res-x Res-y) );q=0.9
         (& (Pix-x=640)  (Pix-y=480) (Res Res-x Res-y) );q=0.8 )
      where
      (Res Res-x Res-y) :-
         (| (& (Res-x=150) (Res-y=150) )
            (& (Res-x=150) (Res-y=300) )
            (& (Res-x=300) (Res-y=300) )
            (& (Res-x=300) (Res-y=600) )
            (& (Res-x=600) (Res-y=600) ) )
      end

   Note that the formal parameters of "Res", "Res-x" and "Res-y",
   prevent the body of the named predicate from referencing similarly-
   named feature values.

6.2 Unit designations

   In some exceptional cases, there may be differing conventions for the
   units of measurement of a given feature.  For example, resolution is
   commonly expressed as dots per inch (dpi) or dots per centimetre
   (dpcm) in different applications (e.g. printing vs faxing).

   In such cases, a unit designator may be appended to a feature value
   according to the conventions indicated below (see also [3]).  These
   considerations apply only to features with numeric values.

   Every feature tag has a standard unit of measurement.  Any expression
   of a feature value that uses this unit is given without a unit
   designation -- this is the normal case.  When the feature value is
   expressed in some other unit, a unit designator is appended to the
   numeric feature value.

   The registration of a feature tag indicates the standard unit of
   measurement for a feature, and also any alternate units and
   corresponding unit designators that may be used, according to RFC
   2506 [3].

   Thus, if the standard unit of measure for resolution is 'dpcm', then
   the feature predicate '(res=200)' would be used to indicate a
   resolution of 200 dots-per-centimetre, and '(res=72dpi)' might be
   used to indicate 72 dots-per-inch.




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   Unit designators are accommodated by the following extension to the
   feature predicate syntax:

      fvalue     =/ number *WSP token

   When performing feature set matching, feature comparisons with and
   without unit designators, or feature comparisons with different unit
   designators, are treated as if they were different features.  Thus,
   the feature predicate '(res=200)' would not, in general, fail to
   match with the predicate '(res=200dpi)'.

      NOTE:  A protocol processor with specific knowledge of the feature
      and units concerned might recognize the relationship between the
      feature predicates in the above example, and fail to match these
      predicates.

      This appears to be a natural behaviour in this simple example, but
      can cause additional complexity in more general cases.
      Accordingly, this is not considered to be required or normal
      behaviour.  It is presumed that an application concerned will
      ensure consistent feature processing by adopting a consistent unit
      for any given feature.

6.3 Unknown feature value data types

   This memo has dealt with feature values that have well-understood
   comparison properties: numbers, with equality, less-than, greater-
   than relationships, and other values with equality relationships
   only.

   Some feature values may have comparison operations that are not
   covered by this framework.  For example, strings containing multi-
   part version numbers: "x.y.z".  Such feature comparisons are not
   covered by this memo.

   Specific applications may recognize and process feature tags that are
   associated with such values.  Future work may define ways to
   introduce new feature value data types in a way that allows them to
   be used by applications that do not contain built-in knowledge of
   their properties.

7. Examples and additional comments

7.1 Worked example

   This example considers sending a document to a high-end black-and-
   white fax system with the following receiver capabilities:




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      (& (dpi=[200,300])
         (grey=2) (color=0)
         (image-coding=[MH,MR]) )

   Turning to the document itself, assume it is available to the sender
   in three possible formats, A4 high resolution, B4 low resolution and
   A4 high resolution colour, described by:

      (& (dpi=300)
         (grey=2)
         (image-coding=MR) )

      (& (dpi=200)
         (grey=2)
         (image-coding=[MH,MMR]) )

      (& (dpi=300) (dpi-xyratio=1)
         (color<=256)
         (image-coding=JPEG) )

   These three image formats can be combined into a composite capability
   statement by a logical-OR operation (to describe format-1 OR format-2
   OR format-3):

      (| (& (dpi=300)
            (grey=2)
            (image-coding=MR) )
         (& (dpi=200)
            (grey=2)
            (image-coding=[MH,MMR]) )
         (& (dpi=300)
            (color<=256)
            (image-coding=JPEG) ) )

   The composite document description can be matched with the receiver
   capability description by combining the capability descriptions with
   a logical AND operation:

      (& (& (dpi=[200,300])
              (grey=2) (color=0)
            (image-coding=[MH,MR]) )
         (| (& (dpi=300)
               (grey=2)
               (image-coding=MR) )
            (& (dpi=200)
               (grey=2)
               (image-coding=[MH,MMR]) )
            (& (dpi=300)



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               (color<=256)
               (image-coding=JPEG) ) ) )

   -->  Expand value-set notation:

      (& (& (| (dpi=200) (dpi=300) )
            (grey=2) (color=0)
            (| (image-coding=MH) (image-coding=MR) ) )
         (| (& (dpi=300)
               (grey=2)
               (image-coding=MR) )
            (& (dpi=200)
               (grey=2)
               (| (image-coding=MH) (image-coding=MMR) ) )
            (& (dpi=300)
               (color<=256)
               (image-coding=JPEG) ) ) )

   -->  Flatten nested '(&...)':

      (& (| (dpi=200) (dpi=300) )
         (grey=2) (color=0)
         (| (image-coding=MH) (image-coding=MR) )
         (| (& (dpi=300)
               (grey=2)
               (image-coding=MR) )
            (& (dpi=200)
               (grey=2)
               (| (image-coding=MH) (image-coding=MMR) ) )
            (& (dpi=300)
               (color<=256)
               (image-coding=JPEG) ) ) )

   -->  (distribute '(&...)' over inner '(|...)'):

      (& (| (dpi=200) (dpi=300) )
         (grey=2) (color=0)
         (| (image-coding=MH) (image-coding=MR) )
         (| (& (dpi=300) (grey=2) (image-coding=MR) )
            (& (dpi=200) (grey=2) (image-coding=MH) )
            (& (dpi=200) (grey=2) (image-coding=MMR) )
            (& (dpi=300) (color<=256) (image-coding=JPEG) ) ) )

   -->  continue to distribute '(&...)' over '(|...)', and flattening
        nested '(&...)' and '(|...)' ...:

      (| (& (dpi=200) (grey=2) (color=0) (image-coding=MH)
            (| (& (dpi=300) (grey=2) (image-coding=MR) )



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               (& (dpi=200) (grey=2) (image-coding=MH) )
               (& (dpi=200) (grey=2) (image-coding=MMR) )
               (& (dpi=300) (color<=256) (image-coding=JPEG) ) ) )
         (& (dpi=200) (grey=2) (color=0) (image-coding=MR)
            (| (& (dpi=300) (grey=2) (image-coding=MR) )
               (& (dpi=200) (grey=2) (image-coding=MH) )
               (& (dpi=200) (grey=2) (image-coding=MMR) )
               (& (dpi=300) (color<=256) (image-coding=JPEG) ) ) )
         (& (dpi=300) (grey=2) (color=0) (image-coding=MH)
            (| (& (dpi=300) (grey=2) (image-coding=MR) )
               (& (dpi=200) (grey=2) (image-coding=MH) )
               (& (dpi=200) (grey=2) (image-coding=MMR) )
               (& (dpi=300) (color<=256) (image-coding=JPEG) ) ) )
         (& (dpi=300) (grey=2) (color=0) (image-coding=MR)
            (| (& (dpi=300) (grey=2) (image-coding=MR) )
               (& (dpi=200) (grey=2) (image-coding=MH) )
               (& (dpi=200) (grey=2) (image-coding=MMR) )
               (& (dpi=300) (color<=256) (image-coding=JPEG) ) ) ) )

   -->  ... until normal form is achieved:

      (| (& (dpi=200) (grey=2) (color=0) (image-coding=MH)