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 (!container.empty() && !comma_p.ignore(pos, expected))
            return false;
        if (!literal_p.parse(pos, literal, expected))
            return false;
        if (!colon_p.parse(pos, literal, expected))
            return false;
        container.push_back(std::move(literal->as<ASTLiteral &>().value));
        if (literal_p.parse(pos, literal, expected))
            container.push_back(std::move(literal->as<ASTLiteral &>().value));
        else
            return parseAllCollectionsStart(pos, collections, expected);
    }
    return true;
}
}
bool ParserAllCollectionsOfLiterals::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    Collections collections;
    if (!parseAllCollectionsStart(pos, collections, expected))
        return false;
    while (!collections.empty())
    {
        if (!collections.back()->parse(pos, collections, node, expected))
            return false;
        if (node)
            collections.pop_back();
    }
    return true;
}
bool ParserLiteral::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)