NLTK: Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()`

SummaryXMLCorpusView.readxmlfragment reads a corpus file in 1 KiB blocks, appendingeach block to a growing fragment string, then calls VALIDXMLRE.matchfragmenton the full accumulated buffer every iteration. Because each iteration rescans theentire accumulated fragment, the total amount of work...