-- Lean 4 proof step from Mathlib.Algebra.Order.Module.HahnEmbedding (goal)
Goal:
K : Type u_1
inst : DivisionRing K
inst_1 : LinearOrder K
inst_2 : IsOrderedRing K
inst_3 : Archimedean K
M : Type u_2
inst_4 : AddCommGroup M
inst_5 : LinearOrder M
inst_6 : IsOrderedAddMonoid M
inst_7 : Module K M
inst_8 : IsOrderedModule K M
R : Type u_3
inst_9 : AddCommGroup R
inst_10 : LinearOrder R
inst_11 : Module K R
seed : Seed K M R
f : Partial seed
inst_12 : IsOrderedAddMonoid R
inst_13 : Archimedean R
x : M
hx : x ∉ (↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain
c : FiniteArchimedeanClass M
y : ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain
hy : Membership.mem (γ := Submodule K M) (ball K c) ((↑y : M) - x)
z : ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain
hz :
  (↑(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧) :
        ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain → Lex R⟦FiniteArchimedeanClass M⟧)
      z =
    (toLex : (a : R⟦FiniteArchimedeanClass M⟧) → Lex R⟦FiniteArchimedeanClass M⟧)
      ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
        ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
          ((↑(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧) :
              ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain → Lex R⟦FiniteArchimedeanClass M⟧)
            y)))
d : FiniteArchimedeanClass M
pos :
  ∀ (hdc : d < c),
    Eq.{u_3 + 1} (α := R)
      (coeff
        ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
          ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
            ((↑(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧) :
                ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain → Lex R⟦FiniteArchimedeanClass M⟧)
              y)))
        d)
      (coeff
        ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
          ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧) (f.eval x)))
        d)
⊢ Eq.{u_3 + 1} (α := R)
    (coeff
      ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
        ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
          ((↑(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧) :
              ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain → Lex R⟦FiniteArchimedeanClass M⟧)
            y)))
      d)
    (coeff
      ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
        ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧) (f.eval x)))
      d)
have pos₁ : ∀ (hdc : c ≤ d),
  Eq.{u_3 + 1} (α := R)
    (coeff
      ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
        ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
          ((↑(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧) :
              ↥(↑f : M →ₗ.[K] Lex R⟦FiniteArchimedeanClass M⟧).domain → Lex R⟦FiniteArchimedeanClass M⟧)
            y)))
      d)
    (coeff
      ((truncLTLinearMap K c : (a : R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧)
        ((ofLex : (a : Lex R⟦FiniteArchimedeanClass M⟧) → R⟦FiniteArchimedeanClass M⟧) (f.eval x)))
      d)
Tactic: obtain hdc | hdc := lt_or_ge d c