English
Let a ∈ Part α with a.Dom, then getOrElse a d = a.get h for h : a.Dom.
Русский
Пусть a ∈ Part α и a.Dom; тогда getOrElse a d = a.get h для некоторого h : a.Dom.
LaTeX
$$$ \forall a : \mathrm{Part}(\alpha),\ h : a.\\Dom, \\forall d : \alpha, \\operatorname{getOrElse}(a,d) = a.\\get(h) $$$
Lean4
theorem getOrElse_of_dom (a : Part α) (h : a.Dom) [Decidable a.Dom] (d : α) : getOrElse a d = a.get h :=
dif_pos h