function _check ($from_lines, $to_lines) {
    	if (serialize($from_lines) != serialize($this->orig()))
    		trigger_error("Reconstructed original doesn't match", E_USER_ERROR);
    	if (serialize($to_lines) != serialize($this->closing()))
    		trigger_error("Reconstructed closing doesn't match", E_USER_ERROR);

    	$rev = $this->reverse();
    	if (serialize($to_lines) != serialize($rev->orig()))
    		trigger_error("Reversed original doesn't match", E_USER_ERROR);
    	if (serialize($from_lines) != serialize($rev->closing()))
    		trigger_error("Reversed closing doesn't match", E_USER_ERROR);


    	$prevtype = 'none';
    	foreach ($this->edits as $edit) {
    		if ( $prevtype == $edit->type )
    			trigger_error("Edit sequence is non-optimal", E_USER_ERROR);
    		$prevtype = $edit->type;
    	}

    	$lcs = $this->lcs();
    	trigger_error("WikiDiff okay: LCS = $lcs", E_USER_NOTICE);
    }