Lean4
/-- `InfSet` structure on a nonempty subset `s` of a preorder with `InfSet`. This definition is
non-canonical (it uses `default s`); it should be used only as here, as an auxiliary instance in the
construction of the `ConditionallyCompleteLinearOrder` structure. -/
noncomputable def subsetInfSet [Inhabited s] : InfSet s where
sInf
t :=
if ht : t.Nonempty ∧ BddBelow t ∧ sInf ((↑) '' t : Set α) ∈ s then ⟨sInf ((↑) '' t : Set α), ht.2.2⟩ else default