+ /**
+ * Renders a parse-tree into a Yaz-style PQF string.
+ * <P>
+ * <PRE>
+ query ::= top-set query-struct.
+ top-set ::= [ '@attrset' string ]
+ query-struct ::= attr-spec | simple | complex | '@term' term-type
+ attr-spec ::= '@attr' [ string ] string query-struct
+ complex ::= operator query-struct query-struct.
+ operator ::= '@and' | '@or' | '@not' | '@prox' proximity.
+ simple ::= result-set | term.
+ result-set ::= '@set' string.
+ term ::= string.
+ proximity ::= exclusion distance ordered relation which-code unit-code.
+ exclusion ::= '1' | '0' | 'void'.
+ distance ::= integer.
+ ordered ::= '1' | '0'.
+ relation ::= integer.
+ which-code ::= 'known' | 'private' | integer.
+ unit-code ::= integer.
+ term-type ::= 'general' | 'numeric' | 'string' | 'oid' | 'datetime' | 'null'.
+ * </PRE>
+ * @return
+ * A String containing a PQF query equivalent to the parse-tree
+ * whose root is this node. This may be fed into the tool of
+ * your choice to obtain a BER-encoded packet.
+ */
+ abstract public String toPQF(Properties config)
+ throws PQFTranslationException;
+
+ /**
+ * Returns a String of spaces for indenting to the specified level.
+ */