English
The operation erase s a returns the Finset s with a removed, i.e., s minus {a}.
Русский
Операция erase s a возвращает Finset s без элемента a, то есть s минус {a}.
LaTeX
$$$ \text{erase}(s,a) = s \setminus \{a\} $$$
Lean4
/-- `erase s a` is the set `s - {a}`, that is, the elements of `s` which are
not equal to `a`. -/
def erase (s : Finset α) (a : α) : Finset α :=
⟨_, s.2.erase a⟩