in general one doesn't want to materialize the intermediates. one evaluation strategy is treat the sets as streams. this removes the for loops, and leaves the e

in general one doesn't want to materialize the intermediates. one evaluation strategy is treat the sets as streams. this removes the for loops, and leaves the evaluation strategy intact. for more complicated expressions, also allows you to move the filters first (referential transparency for the win), just like you would if you were compiling a relational query.