"There are universal types; there are things we know we know. We also know there are existential types; that is to say we know there are some things we do not know. But there are also dynamic types; the ones we don't know we don't know."
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The interesting case is the middle one: existential types, or known unknowns. An instance of an existential type represents evidence that certain facts were known at the time of its creation, but those facts are no longer known at the point we're considering it.
