Flow\Parser::parseSubExpression PHP Method

    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;
    }