Pros:
- it is convenient to type and read numbers this way;
- Ada, Perl, C++, Go and Rust already support that;
- can support thousands as well as lakhs and crores;

Cons:
- standard SQL does not have this support;
- PostgreSQL treat `SELECT 1_000_000` as number one with `_000_000` alias: http://sqlfiddle.com/#!17/0a28f/400
- C++ has `1'000'000` instead of `1_000_000`;

What do you think?

{
    if (pos->type != TokenType::DoubleAt)
        return false;
    ++pos;
    if (pos->type != TokenType::BareWord)
    {
        expected.add(pos, "variable name");
        return false;
    }
    String name(pos->begin, pos->end);
    ++pos;
    
    if (pos->type == TokenType::Dot)
    {
        ++pos;
        if (pos->type != TokenType::BareWord)
        {
            expected.add(pos, "variable name");
            return false;
        }
        name = String(pos->begin, pos->end);
        ++pos;
    }
    auto name_literal = std::make_shared<ASTLiteral>(name);
    node = makeASTFunction("globalVariable", name_literal);
    node->setAlias("@@" + name);
    return true;
}
bool ParserWithOptionalAlias::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)