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

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

{
    if (pos->type != TokenType::OpeningCurlyBrace)
        return false;
    ++pos;
    if (pos->type != TokenType::BareWord)
    {
        expected.add(pos, "substitution name (identifier)");
        return false;
    }
    String name(pos->begin, pos->end);