¬[∀hat ∊ hats, IsGreen(hat)] ⇔ ∃hat ∊ hats, ¬IsGreen(hat)
This reads: for it to not be true that every hat in the collection of hats is green, there must exist at least one hat in the collection of hats that is not green.
alt Hacker News
This reads: for it to not be true that every hat in the collection of hats is green, there must exist at least one hat in the collection of hats that is not green.