looking at documentation again.
one very minor question (issue)
+ <para>
+ The <literal>ON EMPTY</literal> clause specifies the behavior if the
+ <replaceable>path_expression</replaceable> yields no value at all; the
+ default when <literal>ON EMPTY</literal> is not specified is to return
+ a null value.
+ </para>
I think it should be:
applying <replaceable>path_expression</replaceable>
or
evaluating <replaceable>path_expression</replaceable>
not "the <replaceable>path_expression</replaceable>"
?