the value is used by the parser context; the old code called it only after the *first* parser context
was created. therefore the first XMLTree::read() call has its behavior determined by libxml2's default
for this value, rather than by our explicit choice (do not treat whitespace as a note). All subequent
read() calls will use our value.
This variable inside libxml2 is actually per-thread, which just increases the stakes for calling
xmlKeepBlanksDefault() at the right time
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