protected function parseSubExpression()
{
$line = $this->stream->getCurrentToken()->getLine();
$left = $this->parseMulExpression();
while ($this->stream->consume(Token::OPERATOR, '-')) {
$right = $this->parseMulExpression();
$left = new Expression\SubExpression($left, $right, $line);
$line = $this->stream->getCurrentToken()->getLine();
}
return $left;
}