public function prepend($xpath, $prefix)
    {
        $expressions = array();

        // If the xpath prefix contains a union we need to wrap it in parentheses.
        if (preg_match(self::UNION_PATTERN, $prefix)) {
            $prefix = '('.$prefix.')';
        }

        // Split any unions into individual expressions.
        foreach ($this->splitUnionParts($xpath) as $expression) {
            $expression = trim($expression);
            $parenthesis = '';

            // If the union is inside some braces, we need to preserve the opening braces and apply
            // the prefix only inside it.
            if (preg_match('/^[\(\s*]+/', $expression, $matches)) {
                $parenthesis = $matches[0];
                $expression = substr($expression, strlen($parenthesis));
            }

            // add prefix before element selector
            if (0 === strpos($expression, '/')) {
                $expression = $prefix.$expression;
            } else {
                $expression = $prefix.'/'.$expression;
            }
            $expressions[] = $parenthesis.$expression;
        }

        return implode(' | ', $expressions);
    }