I mentioned earlier that Sequel code is generally very concise.
One of the tricks Sequel uses to keep code concise is that it allows you to use a single symbol to contain both a table and a column, by separating them with a double underscore.
It allows a single symbol to contain both a column and an alias by separating them with a triple underscore.
It also allows you to combine the two approaches by using both a double underscore and a triple underscore in the same symbol.