From type/Iterator

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method push-until-lazy(Iterator:D: $target --> Mu)

Should produce values until it considers itself to be lazy, and push them onto $target.

The Iterator role implements this method as a no-op if is-lazy returns a True value, or as a synonym of push-all if not.

This matters mostly for iterators that have other iterators embedded, some of which might be lazy, while others aren't.