Comment by runeks

> Another suggestion around `let`: consider splitting it into two operations, for creating a new column and for modifying an existing one. E.g. called `let` and `set`.

Couldn’t we just not allow modifying an existing column? Ie. we would not allow

  count = count + 1

But force the use of a new variable name:

  new_count = count + 1

I think this makes for much more readable code, since the value of a variable does not depend on line number.