**Use case**
`SELECT POSITION('/' IN s) FROM (SELECT 'Hello/World' AS s);`

The same as `position(s, '/')`

{
    ASTPtr contents_node;
    ParserExpressionList contents(false);
    if (pos->type != TokenType::OpeningRoundBracket)
        return false;
    ++pos;
    if (!contents.parse(pos, contents_node, expected))
        return false;
    bool is_elem = true;
    if (pos->type == TokenType::Comma)
    {
        is_elem = false;
        ++pos;
    }
    if (pos->type != TokenType::ClosingRoundBracket)
        return false;
    ++pos;
    const auto & expr_list = contents_node->as<ASTExpressionList &>();
    
    if (expr_list.children.empty())
    {
        expected.add(pos, "non-empty parenthesized list of expressions");
        return false;
    }
    if (expr_list.children.size() == 1 && is_elem)
    {
        node = expr_list.children.front();