∀ {h : ℝ} {f : UpperHalfPlane → ℂ},
  ContinuousAt (SlashInvariantFormClass.cuspFunction h f) 0 →
    ∀ (a : ℂ), SlashInvariantFormClass.cuspFunction h (a • f) = a • SlashInvariantFormClass.cuspFunction h f