protected function blockHeader( $xbeg, $xlen, $ybeg, $ylen ) {
// '<!--LINE \d+ -->' get replaced by a localised line number
// in DifferenceEngine::localiseLineNumbers
protected function blockHeader( $xbeg, $xlen, $ybeg, $ylen ) {
// '<!--LINE \d+ -->' get replaced by a localised line number
// in DifferenceEngine::localiseLineNumbers