English
FromRel of Ne (not equal) with symmetry yields all non-diagonal pairs.
Русский
FromRel, задаваемое relation Ne, даёт все вне диагонали пары.
LaTeX
$$$$ \mathrm{fromRel}(\mathrm{z.symm}: \mathrm{Symmetric\ } \mathrm{Ne}) = \{ z \mid \neg \mathrm{IsDiag}(z) \}. $$$$
Lean4
theorem fromRel_ne : fromRel (fun (_ _ : α) z => z.symm : Symmetric Ne) = {z | ¬IsDiag z} := by ext z;
exact z.ind (by simp)