{Γ : Subgroup (GL (Fin 2) ℝ)} →
  {k : ℤ} →
    (toFun : UpperHalfPlane → ℂ) →
      (slash_action_eq' : ∀ γ ∈ Γ, SlashAction.map k γ toFun = toFun) →
        MDiff ⇑{ toFun := toFun, slash_action_eq' := slash_action_eq' } →
          (∀ {c : OnePoint ℝ}, IsCusp c Γ → c.IsBoundedAt toFun k) → ModularForm Γ k