diff --git a/coq/_CoqProject b/coq/_CoqProject
index ed0946f..45553df 100644
--- a/coq/_CoqProject
+++ b/coq/_CoqProject
@@ -7,6 +7,7 @@ Atom.v
 Metatheory.v
 
 terms_debruijn.v
+equiv_debruijn.v
 subst_lemmas_debruijn.v
 
 terms.v
diff --git a/coq/equiv_debruijn.v b/coq/equiv_debruijn.v
new file mode 100644
index 0000000..0feaebf
--- /dev/null
+++ b/coq/equiv_debruijn.v
@@ -0,0 +1,180 @@
+Require Import terms_debruijn.
+
+Open Scope ladder_type_scope.
+Open Scope ladder_expr_scope.
+
+Reserved Notation "S '-->distribute-ladder' T" (at level 40).
+
+Inductive type_distribute_ladder : type_DeBruijn -> type_DeBruijn -> Prop :=
+  | L_DistributeOverSpec1 : forall x x' y,
+    [<  <x~x' y>  >]
+    -->distribute-ladder
+    [<  <x y>~<x' y>  >]
+
+  | L_DistributeOverSpec2 : forall x y y',
+    [<  <x y~y'>  >]
+    -->distribute-ladder
+    [<  <x y>~<x y'>  >]
+
+  | L_DistributeOverFun1 : forall x x' y,
+    [<  (x~x' -> y)  >]
+    -->distribute-ladder
+    [<  (x -> y) ~ (x' -> y)  >]
+
+  | L_DistributeOverFun2 : forall x y y',
+    [<  (x -> y~y')  >]
+    -->distribute-ladder
+    [<  (x -> y) ~ (x -> y')  >]
+
+  | L_DistributeOverMorph1 : forall x x' y,
+    [<  (x~x' ->morph y)  >]
+    -->distribute-ladder
+    [<  (x ->morph y) ~ (x' ->morph y)  >]
+
+  | L_DistributeOverMorph2 : forall x y y',
+    [<  x ->morph y~y'  >]
+    -->distribute-ladder
+    [<  (x ->morph y) ~ (x ->morph y')  >]
+
+where "S '-->distribute-ladder' T" := (type_distribute_ladder S T).
+
+
+Reserved Notation "S '-->condense-ladder' T" (at level 40).
+Inductive type_condense_ladder : type_DeBruijn -> type_DeBruijn -> Prop :=
+  | L_CondenseOverSpec1 : forall x x' y,
+    [< <x y>~<x' y> >]
+    -->condense-ladder
+    [< <x~x' y> >]
+
+  | L_CondenseOverSpec2 : forall x y y',
+    [< <x y>~<x y'> >]
+    -->condense-ladder
+    [< <x y~y'> >]
+
+  | L_CondenseOverFun1 : forall x x' y,
+    [< (x -> y) ~ (x' -> y) >]
+    -->condense-ladder
+    [< (x~x') -> y >]
+
+  | L_CondenseOverFun2 : forall x y y',
+    [< (x -> y) ~ (x -> y') >]
+    -->condense-ladder
+    [< (x -> y~y') >]
+
+  | L_CondenseOverMorph1 : forall x x' y,
+    [< (x ->morph y) ~ (x' ->morph y) >]
+    -->condense-ladder
+    [< (x~x' ->morph y) >]
+
+  | L_CondenseOverMorph2 : forall x y y',
+    [< (x ->morph y) ~ (x ->morph y') >]
+    -->condense-ladder
+    [< (x ->morph y~y') >]
+
+where "S '-->condense-ladder' T" := (type_condense_ladder S T).
+
+Create HintDb type_eq_hints.
+
+Hint Constructors type_distribute_ladder : type_eq_hints.
+Hint Constructors type_condense_ladder : type_eq_hints.
+
+(** Inversion Lemma:
+   `-->distribute-ladder` is the inverse of `-->condense-ladder
+*)
+Lemma distribute_inverse :
+  forall x y,
+  x -->distribute-ladder y ->
+  y -->condense-ladder x.
+Proof.
+  intros.
+  destruct H.
+  all: auto with type_eq_hints.
+Qed.
+
+(** Inversion Lemma:
+    `-->condense-ladder`  is the inverse of  `-->distribute-ladder`
+*)
+Lemma condense_inverse :
+  forall x y,
+  x -->condense-ladder y ->
+  y -->distribute-ladder x.
+Proof.
+  intros.
+  destruct H.
+  all: auto with type_eq_hints.
+Qed.
+
+
+(** Define the equivalence relation as reflexive, transitive hull. $\label{coq:type-equiv}$ *)
+
+Reserved Notation " S '===' T " (at level 40).
+Inductive type_eq : type_DeBruijn -> type_DeBruijn -> Prop :=
+  | TEq_Refl : forall x,
+    x === x
+
+  | TEq_Trans : forall x y z,
+    x === y ->
+    y === z ->
+    x === z
+
+  | TEq_SubFun : forall x x' y y',
+    x === x' ->
+    y === y' ->
+    [< x -> y >] === [< x' -> y' >]
+
+  | TEq_SubMorph : forall x x' y y',
+    x === x' ->
+    y === y' ->
+    [< x ->morph y >] === [< x' ->morph y' >]
+
+  | TEq_LadderAssocLR : forall x y z,
+    [< (x~y)~z >]
+    ===
+    [< x~(y~z) >]
+
+  | TEq_LadderAssocRL : forall x y z,
+    [< x~(y~z) >]
+    ===
+    [< (x~y)~z >]
+
+  | TEq_Distribute : forall x y,
+    x -->distribute-ladder y ->
+    x === y
+
+  | TEq_Condense : forall x y,
+    x -->condense-ladder y ->
+    x === y
+
+where "S '===' T" := (type_eq S T).
+
+
+
+(** Symmetry of === *)
+Lemma TEq_Symm :
+  forall x y,
+  (x === y) -> (y === x).
+Proof.
+  intros.
+  induction H.
+
+  apply TEq_Refl.
+
+  apply TEq_Trans with (y:=y).
+  apply IHtype_eq2.
+  apply IHtype_eq1.
+
+  apply TEq_SubFun.
+  auto. auto.
+  apply TEq_SubMorph.
+  auto. auto.
+  apply TEq_LadderAssocRL.
+  apply TEq_LadderAssocLR.
+
+  apply TEq_Condense.
+  apply distribute_inverse.
+  apply H.
+
+  apply TEq_Distribute.
+  apply condense_inverse.
+  apply H.
+Qed.