for

(for [binding...] expr)

Answers expr over every combination of the bindings, left to right. Pairs, patterns and modifiers work like doseq's: each :when skips its element, each :while ends its level, each :let binds sequentially. While every collection it steps over is strict the answer is realized at once, a strict list; from the first lazy collection on (the first binding's included) the rest is a lazy seq, realized as it is consumed, so first and take realize only what they answer and an infinite collection ends behind them.

Deviation: an empty strict result is nil, where the oracle prints (); a strict answer realizes when the for runs, where the oracle's waits to be consumed.