theorem instFiatLineAtToParent
theorem instFiatLineAtToParent: ∀ (x t : Type) [FiatLine.At x t], ContinuantFiatBoundary.At x t
theorem instFiatLineAtToParent
theorem instFiatLineAtToParent: ∀ (x t : Type) [FiatLine.At x t], ContinuantFiatBoundary.At x t