+ query ::= ['@attrset' <attribute-set>] rpn
+ rpn ::= complex | simple | attr rpn
+ complex ::= operator rpn rpn
+ operator ::= '@and' | '@or' | '@not' | '@prox' proximity
+ simple ::= term | set.
+ term ::= <word> | '"' <term> ... '"'.
+ set ::= '@set' <setname>.
+ attr ::= '@attr' [<attribute-set>] <number> '=' <number>.
+ proximity ::= exclusion distance ordered relation which unit
+ exclusion ::= '1' | '0' | 'void'
+ distance ::= <number>
+ ordered ::= '1' | '0'
+ relation ::= <number>
+ which ::= 'known' | 'private'
+ unit ::= <number>
+
+Eg.:
+
+ Andersen
+
+ "Hans Andersen"
+
+ @and @attr 1=1 andersen @attr 1=4 @attr 4=2 duckling
+
+ @attrset Bib-1 @and @attr GILS 2=2008 Washington @attr 1=21 weather
+
+The sort command takes a sequence of sort specifications. A sort
+specification holds a field (sort criteria) and is followed by flags.
+If the sort critieria includes = it is assumed that the sort SortKey
+is of type sortAttributes using Bib-1. The integer before the = is
+the attribute type and the integer following the = is the attribute
+value. If no = is in the SortKey it is treated as a sortfield-type
+of type InternationalString. Flags observed are s (sort case
+sensitive), i (sort case insensitive), < (ascending), > (descending).
+Eg.:
+ 1=4 i< (use is title, insensitive, ascending).
+ Title s> (String Title, sensitive, descending).