diff --git a/Skolem.lp b/Skolem.lp
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+++ b/Skolem.lp
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+require open Stdlib.Prop Stdlib.FOL Stdlib.Epsilon;
+
+// procedure for skolemizing all existentials (no proof provided)
+// /!\ WARNING: this is a sequential symbol
+
+private sequential symbol skolem: Prop → Prop;
+
+rule skolem (∃ $p) ↪ skolem ($p (ε $p))
+with skolem (∀ $p) ↪ `∀ x, skolem ($p x)
+with skolem ($p ∧ $q) ↪ skolem $p ∧ skolem $q
+with skolem ($p ∨ $q) ↪ skolem $p ∨ skolem $q
+with skolem $p ↪ $p
+;
+
+// example
+
+require open Stdlib.Set Stdlib.Nat;
+
+private symbol P:ℕ → TYPE;
+
+rule P 0 ↪ Prop
+with P ($n +1) ↪ τ ι → P $n;
+
+private symbol p : P 2;
+private symbol q : P 4;
+
+private symbol A ≔ `∀ X1, `∃ X2, p X1 X2 ∧ `∀ X3, `∃ X4, q X1 X2 X3 X4;
+
+private symbol sk1 X1 ≔ ε (λ X2, p X1 X2 ∧ `∀ X3, `∃ X4, q X1 X2 X3 X4);
+
+private symbol B ≔ `∀ X1, p X1 (sk1 X1) ∧ `∀ X3, `∃ X4, q X1 (sk1 X1) X3 X4;
+
+private symbol sk2 X1 X3 ≔ ε (λ X4, q X1 (sk1 X1) X3 X4);
+
+private symbol C ≔ `∀ X1, p X1 (sk1 X1) ∧ `∀ X3, q X1 (sk1 X1) X3 (sk2 X1 X3);
+
+assert ⊢ skolem A ≡ C;
+
+// propositional atoms
+
+symbol at:ℕ → Prop;
+
+// we assume that Prop is inductive
+
+symbol ind_Prop (p:Prop → Prop):
+(Π k, π(p (at k))) →
+(Π a f, π((`∀ x:τ a, p (f x)) ⇒ p (∃ f))) →
+(Π a f, π((`∀ x:τ a, p (f x)) ⇒ p (∀ f))) →
+(Π f g, π(p f ⇒ p g ⇒ p (f ∧ g))) →
+(Π f g, π(p f ⇒ p g ⇒ p (f ∨ g))) →
+Π f:Prop, π(p f);
+
+// positions in a proposition
+
+inductive Pos : TYPE ≔
+| top: Pos
+| all: Pos → Pos
+| ex: Pos → Pos
+| and1: Pos → Pos
+| and2: Pos → Pos
+| or1: Pos → Pos
+| or2: Pos → Pos
+;
+
+// valid positions
+
+symbol valid: Pos → Prop → Prop;
+
+rule valid top (at _) ↪ ⊥
+with valid top (∃ _) ↪ ⊤
+with valid top (∀ _) ↪ ⊥
+with valid top (_ ∧ _) ↪ ⊥
+with valid top (_ ∨ _) ↪ ⊥
+with valid top (_ ⇒ _) ↪ ⊥
+
+with valid (all _) (at _) ↪ ⊥
+with valid (all _) (∃ _) ↪ ⊥
+with valid (all $i) (∀ $f) ↪ `∀ x, valid $i ($f x)
+with valid (all _) (_ ∧ _) ↪ ⊥
+with valid (all _) (_ ∨ _) ↪ ⊥
+with valid (all _) (_ ⇒ _) ↪ ⊥
+
+with valid (ex _) (at _) ↪ ⊥
+with valid (ex $i) (∃ $f) ↪ `∀ x, valid $i ($f x)
+with valid (ex _) (∀ _) ↪ ⊥
+with valid (ex _) (_ ∧ _) ↪ ⊥
+with valid (ex _) (_ ∨ _) ↪ ⊥
+with valid (ex _) (_ ⇒ _) ↪ ⊥
+
+with valid (and1 _) (at _) ↪ ⊥
+with valid (and1 _) (∃ _) ↪ ⊥
+with valid (and1 _) (∀ _) ↪ ⊥
+with valid (and1 $i) ($f ∧ _) ↪ valid $i $f
+with valid (and1 _) (_ ∨ _) ↪ ⊥
+with valid (and1 _) (_ ⇒ _) ↪ ⊥
+
+with valid (and2 _) (at _) ↪ ⊥
+with valid (and2 _) (∃ _) ↪ ⊥
+with valid (and2 _) (∀ _) ↪ ⊥
+with valid (and2 $i) (_ ∧ $f) ↪ valid $i $f
+with valid (and2 _) (_ ∨ _) ↪ ⊥
+with valid (and2 _) (_ ⇒ _) ↪ ⊥
+
+with valid (or1 _) (at _) ↪ ⊥
+with valid (or1 _) (∃ _) ↪ ⊥
+with valid (or1 _) (∀ _) ↪ ⊥
+with valid (or1 _) (_ ∧ _) ↪ ⊥
+with valid (or1 $i) ($f ∨ _) ↪ valid $i $f
+with valid (or1 _) (_ ⇒ _) ↪ ⊥
+
+with valid (or2 _) (at _) ↪ ⊥
+with valid (or2 _) (∃ _) ↪ ⊥
+with valid (or2 _) (∀ _) ↪ ⊥
+with valid (or2 _) (_ ∧ _) ↪ ⊥
+with valid (or2 $i) (_ ∨ $f) ↪ valid $i $f
+with valid (or2 _) (_ ⇒ _) ↪ ⊥
+;
+
+// induction principle on valid positions
+
+require open Stdlib.Impred;
+
+opaque symbol ind_valid (p:Pos → Prop → Prop):
+(Π a (f:τ a → Prop), π(p top (∃ f))) →
+(Π i a (f:τ a → Prop),
+  π((`∀ x, valid i (f x)) ⇒ (`∀ x, p i (f x)) ⇒ p (all i) (∀ f))) →
+(Π i a (f:τ a → Prop),
+  π((`∀ x, valid i (f x)) ⇒ (`∀ x, p i (f x)) ⇒ p (ex i) (∃ f))) →
+(Π i f g, π(valid i f ⇒ p i f ⇒ p (and1 i) (f ∧ g))) →
+(Π i f g, π(valid i g ⇒ p i g ⇒ p (and2 i) (f ∧ g))) →
+(Π i f g, π(valid i f ⇒ p i f ⇒ p (or1 i) (f ∨ g))) →
+(Π i f g, π(valid i g ⇒ p i g ⇒ p (or2 i) (f ∨ g))) →
+Π i f, π(valid i f ⇒ p i f) ≔
+begin
+assume p ptop pall pex pand1 pand2 por1 por2; induction
+// top
+{ refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply ptop }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+}
+// all
+{ assume i hi; refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume a f hf v; apply pall i a f v; assume x; apply hi; refine (v x) }
+  { assume f g hf hg v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+}
+// ex
+{ assume i hi; refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply pex i a f v; assume x; apply hi; refine (v x) }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+}
+// and1
+{ assume i hi; refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply pand1 i f g v (hi f v) }
+  { assume f g hf hg v; apply ⊥ₑ v }
+}
+// and2
+{ assume i hi; refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply pand2 i f g v (hi g v) }
+  { assume f g hf hg v; apply ⊥ₑ v }
+}
+// or1
+{ assume i hi; refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply por1 i f g v (hi f v) }
+}
+// or2
+{ assume i hi; refine ind_Prop _ _ _ _ _ _
+  { assume k v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume a f hf v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply ⊥ₑ v }
+  { assume f g hf hg v; apply por2 i f g v (hi g v) }
+}
+end;
+
+// skolemization of a particular existential
+
+// /!\ WARNING: partial function defined on valid positions only
+symbol skolem1: Pos → Prop → Prop;
+
+rule skolem1 top (∃ $p) ↪ $p (ε $p)
+with skolem1 (all $i) (∀ $p) ↪ `∀ x, skolem1 $i ($p x)
+with skolem1 (ex $i) (∃ $p) ↪ `∃ x, skolem1 $i ($p x)
+with skolem1 (and1 $i) ($p ∧ $q) ↪ skolem1 $i $p ∧ $q
+with skolem1 (and2 $i) ($p ∧ $q) ↪ $p ∧ skolem1 $i $q
+with skolem1 (or1 $i) ($p ∨ $q) ↪ skolem1 $i $p ∨ $q
+with skolem1 (or2 $i) ($p ∨ $q) ↪ $p ∨ skolem1 $i $q
+;
+
+assert ⊢ skolem1 (all top) A ≡ B;
+assert ⊢ skolem1 (all(and2(all top))) B ≡ C;
+
+// skolemization preserves provability
+
+opaque symbol skolem1_preserves_provability i f:
+  π(valid i f) → π f → π(skolem1 i f) ≔
+begin
+refine ind_valid (λ i f, f ⇒ skolem1 i f) _ _ _ _ _ _ _
+{ simplify; assume a f h; apply εᵢ _ h }
+{ simplify; assume i a f v IH h x; apply IH x (h x) }
+{ simplify; assume i a f v IH h; apply ∃ₑ h; assume x hx; apply ∃ᵢ x; apply IH x; apply hx }
+{ simplify; assume i f g v IH h; apply ∧ᵢ { apply IH (∧ₑ₁ h) } { apply ∧ₑ₂ h } }
+{ simplify; assume i f g v IH h; apply ∧ᵢ { apply ∧ₑ₁ h } { apply IH (∧ₑ₂ h) } }
+{ simplify; assume i f g v IH h; apply ∨ₑ h { assume hf; apply ∨ᵢ₁ (IH hf) } { assume hg; apply ∨ᵢ₂ hg } }
+{ simplify; assume i f g v IH h; apply ∨ₑ h { assume hf; apply ∨ᵢ₁ hf } { assume hg; apply ∨ᵢ₂ (IH hg) } }
+end;
+
+opaque symbol skolem1_reflects_provability i f:
+  π(valid i f) → π(skolem1 i f) → π f ≔
+begin
+refine ind_valid (λ i f, skolem1 i f ⇒ f) _ _ _ _ _ _ _
+{ simplify; assume a f h; apply ∃ᵢ _ h }
+{ simplify; assume i a f v IH h x; apply IH x (h x) }
+{ simplify; assume i a f v IH h; apply ∃ₑ h; assume x hx; apply ∃ᵢ x; apply IH x; apply hx }
+{ simplify; assume i f g v IH h; apply ∧ᵢ { apply IH (∧ₑ₁ h) } { apply ∧ₑ₂ h } }
+{ simplify; assume i f g v IH h; apply ∧ᵢ { apply ∧ₑ₁ h } { apply IH (∧ₑ₂ h) } }
+{ simplify; assume i f g v IH h; apply ∨ₑ h { assume hf; apply ∨ᵢ₁ (IH hf) } { assume hg; apply ∨ᵢ₂ hg } }
+{ simplify; assume i f g v IH h; apply ∨ₑ h { assume hf; apply ∨ᵢ₁ hf } { assume hg; apply ∨ᵢ₂ (IH hg) } }
+end;