[xml] xpath and xmlXPathObject::nodesetval, need clarification

Hi. Currently I'm investigating a possible bug in php's simplexml
module, where running certain queries return FALSE instead of an empty
container object. I have tracked it to the following piece of code:

        xmlNodeSetPtr      result;
        xmlXPathObjectPtr  retval;
        retval = xmlXPathEval((xmlChar *)query, sxe->xpath);
        result = retval->nodesetval;
        if (!result) {

Here, not having a 'nodesetval' pointer is treated as an error.
I have found that libxml's xmlXPathEval operation behaves quite
strangely, so I would like to know whether the following text is
intended behavior, and perhaps an explanation of why it is so:

1. xpath('/foo') <- returns nodesetval containing zero nodes
2. xpath('/@attr') <- returns nodesetval containing zero nodes
3. xpath('/foo/bar') <- returns nodesetval containing zero nodes
4. xpath('/foo/@attr') <- returns NULL nodesetval

All three return a result of type XPATH_NODESET. In all three cases,
the elements/attributes queried for are nonexistent.

The scenario occurs when trying to query for an attribute of an
element for which no match exists. A typical example - getting a list
of ids: xpath('/data/entry[$condition]/@id'). If $condition evaluates
to false for all data, I get the above situation.

Comments / suggestions on how to correct the above code are welcome.

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