Notation for `filter.eventually`

Stream: maths

[Yury G. Kudryashov]

In comments to #1897 @Mario Carneiro suggested using (1) ∀ᶠ x ← f, p x for filter.eventually. Currently we use (2) ∀ᶠ x in f, p x. Another possible notation would be (3) ∀ᶠ x → f, p x. Which one do you like more?

[Mario Carneiro]

I'm fine with the right arrow btw