| refunit | (reference unit) names the unit (book, chapter, canto, verse, ...) identified
by this step in a canonical reference. | 
|  | Datatype: CDATA | 
|  | Values: any string of characters; typically a word or phrase in
some natural language. | 
|  | Default: #IMPLIED | 
| Note | The provision of this attribute helps make the structure
of the canonical reference much clearer.
Its use is strongly recommended. | 
| length | specifies the fixed length of the reference component. | 
|  | Datatype: NUMBER | 
|  | Values: Should be a positive integer; if no value is provided,
    the length is unlimited and goes to the next delimiter or to the
    end of the value. | 
|  | Default: #IMPLIED | 
| Note | When constructing a reference, if the reference component
  found is of numeric type, the length
  is made up by inserting leading zeros; if it is not, by inserting
  trailing blanks. In either case, reference components are truncated
  if necessary at the right hand side.
   When seeking a reference, the length indicates the
  number of characters which should be compared. Values longer than
  this will be regarded as matching, if they start correctly.  | 
| delim | supplies a delimiting string following the reference
          component. | 
|  | Datatype: CDATA | 
|  | Values: If a single space is used it is interpreted as
         whitespace | 
|  | Default: #IMPLIED | 
| Note |  | 
| from | specifies the starting point of the area referred to by this
step in the canonical reference. | 
|  | Datatype: %extPtr | 
|  | Values: a valid expression in the TEI extended pointer notation
documented in section 14.2  . | 
|  | Default: #REQUIRED | 
| to | specifies the ending point of the area referred to by this step
in the canonical reference. | 
|  | Datatype: %extPtr | 
|  | Values: a valid expression in the TEI extended pointer notation
documented in section 14.2  . | 
|  | Default: 'DITTO' | 
| Note |  (optional) |