/-- Let `E` and `F` be two topological vector spaces over a `NontriviallyNormedField`, and assume
that the topology of `F` is generated by some family of seminorms `q`. For a family `f` of linear
maps from `E` to `F`, the following are equivalent:
* `f` is equicontinuous at `0`.
* `f` is equicontinuous.
* `f` is uniformly equicontinuous.
* For each `q i`, the family of seminorms `k ↦ (q i) ∘ (f k)` is bounded by some continuous
  seminorm `p` on `E`.
* For each `q i`, the seminorm `⊔ k, (q i) ∘ (f k)` is well-defined and continuous.

In particular, if you can determine all continuous seminorms on `E`, that gives you a complete
characterization of equicontinuity for linear maps from `E` to `F`. For example `E` and `F` are
both normed spaces, you get `NormedSpace.equicontinuous_TFAE`. -/
protected theorem _root_.WithSeminorms.equicontinuous_TFAE {κ : Type*}
    {q : SeminormFamily 𝕜₂ F ι'} [UniformSpace E] [UniformAddGroup E] [u : UniformSpace F]
    [hu : UniformAddGroup F] (hq : WithSeminorms q) [ContinuousSMul 𝕜 E]
    (f : κ → E →ₛₗ[σ₁₂] F) : TFAE
    [ EquicontinuousAt ((↑) ∘ f) 0,
      Equicontinuous ((↑) ∘ f),
      UniformEquicontinuous ((↑) ∘ f),
      ∀ i, ∃ p : Seminorm 𝕜 E, Continuous p ∧ ∀ k, (q i).comp (f k) ≤ p,
      ∀ i, BddAbove (range fun k ↦ (q i).comp (f k)) ∧ Continuous (⨆ k, (q i).comp (f k)) ] := by
  -- We start by reducing to the case where the target is a seminormed space
  rw [q.withSeminorms_iff_uniformSpace_eq_iInf.mp hq, uniformEquicontinuous_iInf_rng,
      equicontinuous_iInf_rng, equicontinuousAt_iInf_rng]
  refine forall_tfae [_, _, _, _, _] fun i ↦ ?_
  let _ : SeminormedAddCommGroup F := (q i).toSeminormedAddCommGroup
  clear u hu hq
  -- Now we can prove the equivalence in this setting
  simp only [List.map]
  tfae_have 1 → 3
  · exact uniformEquicontinuous_of_equicontinuousAt_zero f
  tfae_have 3 → 2
  · exact UniformEquicontinuous.equicontinuous
  tfae_have 2 → 1
  · exact fun H ↦ H 0
  tfae_have 3 → 5
  · intro H
    have : ∀ᶠ x in 𝓝 0, ∀ k, q i (f k x) ≤ 1 := by
      filter_upwards [Metric.equicontinuousAt_iff_right.mp (H.equicontinuous 0) 1 one_pos]
        with x hx k
      simpa using (hx k).le
    have bdd : BddAbove (range fun k ↦ (q i).comp (f k)) :=
      Seminorm.bddAbove_of_absorbent (absorbent_nhds_zero this)
        (fun x hx ↦ ⟨1, forall_range_iff.mpr hx⟩)
    rw [← Seminorm.coe_iSup_eq bdd]
    refine ⟨bdd, Seminorm.continuous' (r := 1) ?_⟩
    filter_upwards [this] with x hx
    simpa only [closedBall_iSup bdd _ one_pos, mem_iInter, mem_closedBall_zero] using hx
  tfae_have 5 → 4
  · exact fun H ↦ ⟨⨆ k, (q i).comp (f k), Seminorm.coe_iSup_eq H.1 ▸ H.2, le_ciSup H.1⟩
  tfae_have 4 → 1 -- This would work over any `NormedField`
  · intro ⟨p, hp, hfp⟩
    exact Metric.equicontinuousAt_of_continuity_modulus p (map_zero p ▸ hp.tendsto 0) _ <|
      eventually_of_forall fun x k ↦ by 

Proof state:
𝕜 : Type u_1
𝕜₂ : Type u_2
𝕝 : Type u_3
𝕝₂ : Type u_4
E : Type u_5
F : Type u_6
G : Type u_7
ι : Type u_8
ι' : Type u_9
inst✝¹⁶ : NontriviallyNormedField 𝕜
inst✝¹⁵ : AddCommGroup E
inst✝¹⁴ : Module 𝕜 E
inst✝¹³ : NormedField 𝕝
inst✝¹² : Module 𝕝 E
inst✝¹¹ : NontriviallyNormedField 𝕜₂
inst✝¹⁰ : AddCommGroup F
inst✝⁹ : Module 𝕜₂ F
inst✝⁸ : NormedField 𝕝₂
inst✝⁷ : Module 𝕝₂ F
σ₁₂ : 𝕜 →+* 𝕜₂
inst✝⁶ : RingHomIsometric σ₁₂
τ₁₂ : 𝕝 →+* 𝕝₂
inst✝⁵ : RingHomIsometric τ₁₂
inst✝⁴ : Nonempty ι
inst✝³ : Nonempty ι'
κ : Type u_10
q : SeminormFamily 𝕜₂ F ι'
inst✝² : UniformSpace E
inst✝¹ : UniformAddGroup E
inst✝ : ContinuousSMul 𝕜 E
f : κ → E →ₛₗ[σ₁₂] F
i : ι'
x✝ : SeminormedAddCommGroup F := AddGroupSeminorm.toSeminormedAddCommGroup (q i).toAddGroupSeminorm
tfae_1_to_3 : EquicontinuousAt (FunLike.coe ∘ f) 0 → UniformEquicontinuous (FunLike.coe ∘ f)
tfae_3_to_2 : UniformEquicontinuous (FunLike.coe ∘ f) → Equicontinuous (FunLike.coe ∘ f)
tfae_2_to_1 : Equicontinuous (FunLike.coe ∘ f) → EquicontinuousAt (FunLike.coe ∘ f) 0
tfae_3_to_5 :
  UniformEquicontinuous (FunLike.coe ∘ f) →
    BddAbove (Set.range fun k => comp (q i) (f k)) ∧ Continuous (⨆ k, ⇑(comp (q i) (f k)))
tfae_5_to_4 :
  BddAbove (Set.range fun k => comp (q i) (f k)) ∧ Continuous (⨆ k, ⇑(comp (q i) (f k))) →
    ∃ p, Continuous ⇑p ∧ ∀ (k : κ), comp (q i) (f k) ≤ p
p : Seminorm 𝕜 E
hp : Continuous ⇑p
hfp : ∀ (k : κ), comp (q i) (f k) ≤ p
x : E
k : κ
⊢ dist ((FunLike.coe ∘ f) k 0) ((FunLike.coe ∘ f) k x) ≤ p x

Next tactic:
simpa using hfp k x