English
Set.insert a s is the set {a} ∪ s.
Русский
Set.insert a s есть множество {a} ∪ s.
LaTeX
$$$\\operatorname{insert}(a,s) = \\{b\\;|\\; b = a \\lor b \\in s\\}$$$
Lean4
/-- `Set.insert a s` is the set `{a} ∪ s`.
Note that you should **not** use this definition directly, but instead write `insert a s` (which is
mediated by the `Insert` typeclass). -/
protected def insert (a : α) (s : Set α) : Set α :=
{b | b = a ∨ b ∈ s}