Use previously defined LET variables in RHS of a LET variable definition

From trevorm@simba.com: "I found that any LET alias that references another will always return MISSING. This is not a high priority for me as I no longer have a use-case that requires it."

We should at least emphasize that it is not supported; the following query will correctly return 0 but the result could be potentially confusing if there is an expectation that previous LET variables can be used in a definition:

SELECT count(beer) 

FROM `beer-sample` beer

LET  min_abv = 5, 

     strong_abv = beer.abv > min_abv * 2

WHERE type = "beer" AND strong_abv