ReadonlycollectionReadonlycondOptional if cond filter clause.
ReadonlygroupObject-form ... grouping marker.
ReadonlyisReadonlykeyObject-form key expression (null for tuple-for).
ReadonlykeyFirst iteration variable (present only for for k, v in ...).
ReadonlykindReadonlypartsReadonlyrangeReadonlyvalueReadonlyvalueSecond (or only) iteration variable — always present.
[for k, v in collection : expr if cond]tuple-for, or{for k, v in collection : key => value ... if cond}object-for.