Although UserLand owns copywrite to the spec, Dave Winer gave us explicit permission to publish separate extensions to that spec in a message on the xml-rpc list.
The specific text below was provided by Skip Montanaro <firstname.lastname@example.org>.
<value> can also be of type
where appropriate to the application.
<nil/> is a discriminated
null value, such as Python's None object, Perl's undef value, or a NULL pointer
Here's an example of a two-element
<struct> that represents
a range. The lowerBound value is an
<int>. The upperBound
<nil/>, indicating that the range has no upper bound.
<struct> <member> <name>lowerBound</name> <value><int>18</int></value> </member> <member> <name>upperBound</name> <value><nil/></value> </member> </struct>
Note that this does not mean that
<nil/> can be substituted
<int> anywhere. It is explicitly the responsibility of
the server author to identify appropriate types for input parameters and return
values. In the example above, if the server author has not indicated the server
is prepared to accept
<nil/> as an alternate to an explicit
<int>, it is an error for the client to pass
In this case the behavior of the server is undefined.
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