diff --git a/src/passes.ml b/src/passes.ml
new file mode 100644
index 0000000..155dd04
--- /dev/null
+++ b/src/passes.ml
@@ -0,0 +1,67 @@
+(** This file contains simplification passes for our Lustre-like AST *)
+
+open Ast
+
+(** [node_pass] is an auxiliary function used to write passes: it will iterate
+  * the function passed as argument on all the nodes of the program *)
+let node_pass f ast: t_nodelist option =
+  Utils.list_map_option f ast
+
+(** [equation_pass] is an auxiliary function used to write passes: it will
+  * iterate the function passed as argument on all the equations of the
+  * program *)
+let equation_pass f ast: t_nodelist option =
+  let aux (node: t_node): t_node option =
+    match Utils.list_map_option f node.n_equations with
+    | None -> None
+    | Some eqs -> Some {n_name         = node.n_name;
+                        n_inputs       = node.n_inputs;
+                        n_outputs      = node.n_outputs;
+                        n_local_vars   = node.n_local_vars;
+                        n_equations    = eqs;
+                        n_inputs_type  = node.n_inputs_type;
+                        n_outputs_type = node.n_outputs_type;
+                       }
+    in
+  node_pass aux ast
+
+let expression_pass f: t_nodelist -> t_nodelist option =
+  let aux (patt, expr) =
+    match f expr with
+    | None -> None
+    | Some expr -> Some (patt, expr)
+  in
+  equation_pass aux
+
+let pre2vars =
+  let rec aux = function
+    | EVar   (ty, v) -> EVar (ty, v)
+    | EMonOp (ty, mop, expr) ->
+               let expr = aux expr in EMonOp (ty, mop, expr)
+    | EBinOp (ty, bop, expr, expr') ->
+        let expr = aux expr in let expr' = aux expr' in
+        EBinOp (ty, bop, expr, expr')
+    | ETriOp (ty, top, expr, expr', expr'') ->
+        let expr = aux expr in let expr' = aux expr' in
+        let expr'' = aux expr'' in
+        ETriOp (ty, top, expr, expr', expr'')
+    | EComp  (ty, cop, expr, expr') ->
+        let expr = aux expr in let expr' = aux expr' in
+        EComp (ty, cop, expr, expr')
+    | EWhen  (ty, expr, expr') ->
+        let expr = aux expr in let expr' = aux expr' in
+        EWhen (ty, expr, expr')
+    | EReset (ty, expr, expr') ->
+        let expr = aux expr in let expr' = aux expr' in
+        EReset (ty, expr, expr')
+    | EConst (ty, c) -> EConst (ty, c)
+    | ETuple (ty, elist) ->
+        let elist =
+          List.fold_right (fun expr acc -> (aux expr) :: acc) elist [] in
+        ETuple (ty, elist)
+    | EApp   (ty, node, arg) ->
+        let arg = aux arg in
+        EApp (ty, node, arg)
+  in
+  expression_pass (Utils.somify aux)
+