-- Module: Mathlib.RingTheory.LaurentSeries
Goal:
K : Type u_2
inst : Field K
S : Set (K⟮X⟯ × K⟮X⟯)
w : Set K⟮X⟯
hT : Membership.mem (γ := Filter K⟮X⟯) (nhds 0) w
pre_T : (fun (x : K⟮X⟯ × K⟮X⟯) ↦ x.2 - x.1) ⁻¹' w ⊆ S
d : (MonoidWithZeroHom.ValueGroup₀ (F := Valuation K⟮X⟯ (WithZero (Multiplicative ℤ))) Valued.v)ˣ
hd :
  {y : K⟮X⟯ |
      ((Valuation.restrict Valued.v :
              Valuation K⟮X⟯
                (MonoidWithZeroHom.ValueGroup₀ (F := K⟮X⟯ →*₀ WithZero (Multiplicative ℤ))
                  (↑Valued.v : K⟮X⟯ →*₀ WithZero (Multiplicative ℤ)))) :
            (a : K⟮X⟯) → MonoidWithZeroHom.ValueGroup₀ (↑Valued.v : K⟮X⟯ →*₀ WithZero (Multiplicative ℤ)))
          (y - 0) <
        (↑d : MonoidWithZeroHom.ValueGroup₀ Valued.v)} ⊆
    w
X : Set K⸨X⸩ :=
  {f : K⸨X⸩ |
    LT.lt (α := WithZero (Multiplicative ℤ))
      (((Valued.v : Valuation K⸨X⸩ (WithZero (Multiplicative ℤ))) : (a : K⸨X⸩) → WithZero (Multiplicative ℤ)) f :
        WithZero (Multiplicative ℤ))
      ((embedding : (a : MonoidWithZeroHom.ValueGroup₀ Valued.v) → WithZero (Multiplicative ℤ))
          (↑d : MonoidWithZeroHom.ValueGroup₀ Valued.v) :
        WithZero (Multiplicative ℤ))}
X_def :
  X =
    {f : K⸨X⸩ |
      LT.lt (α := WithZero (Multiplicative ℤ))
        (((Valued.v : Valuation K⸨X⸩ (WithZero (Multiplicative ℤ))) : (a : K⸨X⸩) → WithZero (Multiplicative ℤ)) f :
          WithZero (Multiplicative ℤ))
        ((embedding : (a : MonoidWithZeroHom.ValueGroup₀ Valued.v) → WithZero (Multiplicative ℤ))
            (↑d : MonoidWithZeroHom.ValueGroup₀ Valued.v) :
          WithZero (Multiplicative ℤ))}
x : K⟮X⟯ × K⟮X⟯
h_1 :
  Membership.mem (γ := Set (K⟮X⟯ × K⟮X⟯))
    (Set.preimage (β := K⸨X⸩ × K⸨X⸩)
      (fun (x : K⟮X⟯ × K⟮X⟯) ↦
        ((algebraMap K⟮X⟯ K⸨X⸩ : (a : K⟮X⟯) → K⸨X⸩) x.1, (algebraMap K⟮X⟯ K⸨X⸩ : (a : K⟮X⟯) → K⸨X⸩) x.2))
      ((fun (x : K⸨X⸩ × K⸨X⸩) ↦ x.2 - x.1) ⁻¹' X))
    x
h :
  LT.lt (α := WithZero (Multiplicative ℤ))
    (((valuation K⸨X⸩ (PowerSeries.idealX K) : Valuation K⸨X⸩ (WithZero (Multiplicative ℤ))) :
          (a : K⸨X⸩) → WithZero (Multiplicative ℤ))
        (HSub.hSub (α := K⸨X⸩) (β := K⸨X⸩) ((algebraMap K⟮X⟯ K⸨X⸩ : (a : K⟮X⟯) → K⸨X⸩) x.2 : K⸨X⸩)
          ((algebraMap K⟮X⟯ K⸨X⸩ : (a : K⟮X⟯) → K⸨X⸩) x.1 : K⸨X⸩)) :
      WithZero (Multiplicative ℤ))
    ((embedding : (a : MonoidWithZeroHom.ValueGroup₀ Valued.v) → WithZero (Multiplicative ℤ))
        (↑d : MonoidWithZeroHom.ValueGroup₀ Valued.v) :
      WithZero (Multiplicative ℤ))
⊢ Membership.mem (γ := Set K⟮X⟯)
    {y : K⟮X⟯ |
      ((Valuation.restrict Valued.v :
              Valuation K⟮X⟯
                (MonoidWithZeroHom.ValueGroup₀ (F := K⟮X⟯ →*₀ WithZero (Multiplicative ℤ))
                  (↑Valued.v : K⟮X⟯ →*₀ WithZero (Multiplicative ℤ)))) :
            (a : K⟮X⟯) → MonoidWithZeroHom.ValueGroup₀ (↑Valued.v : K⟮X⟯ →*₀ WithZero (Multiplicative ℤ)))
          (y - 0) <
        (↑d : MonoidWithZeroHom.ValueGroup₀ Valued.v)}
    ((fun (x : K⟮X⟯ × K⟮X⟯) ↦ x.2 - x.1) x)

Tactic:
rw [Set.mem_setOf_eq, sub_zero, Valuation.restrict_lt_iff_lt_embedding, v_def,
        valuation_eq_LaurentSeries_valuation, map_sub]

Premises used: map_sub, sub_zero, Valuation.restrict_lt_iff_lt_embedding, Set.mem_setOf_eq, RatFunc.valuation_eq_LaurentSeries_valuation, RatFunc.v_def