diff --git a/src/Lean/Elab/PreDefinition/WF/Main.lean b/src/Lean/Elab/PreDefinition/WF/Main.lean
index 57025371b0..5c4cbab688 100644
--- a/src/Lean/Elab/PreDefinition/WF/Main.lean
+++ b/src/Lean/Elab/PreDefinition/WF/Main.lean
@@ -4,12 +4,23 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Leonardo de Moura
 -/
 import Lean.Elab.PreDefinition.Basic
-namespace Lean
-namespace Elab
+import Lean.Elab.PreDefinition.WF.PackDomain
+
+namespace Lean.Elab
+open WF
 open Meta
 
-def wfRecursion (preDefs : Array PreDefinition) (terminationBy? : Option Syntax) : TermElabM Bool :=
+def wfRecursion (preDefs : Array PreDefinition) (terminationBy? : Option Syntax) : TermElabM Bool := do
+  withoutModifyingEnv do
+    for preDef in preDefs do
+      addAsAxiom preDef
+    let unaryPreDefs ← packDomain preDefs
+    for preDef in unaryPreDefs do
+      check preDef.value -- TODO: remove
+      trace[Elab.definition.wf] "{preDef.declName}, {preDef.levelParams}, {preDef.value}"
+  -- TODO
   throwError "well founded recursion has not been implemented yet"
 
-end Elab
-end Lean
+builtin_initialize registerTraceClass `Elab.definition.wf
+
+end Lean.Elab
diff --git a/src/Lean/Elab/PreDefinition/WF/PackDomain.lean b/src/Lean/Elab/PreDefinition/WF/PackDomain.lean
new file mode 100644
index 0000000000..ef288e1899
--- /dev/null
+++ b/src/Lean/Elab/PreDefinition/WF/PackDomain.lean
@@ -0,0 +1,106 @@
+/-
+Copyright (c) 2021 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Leonardo de Moura
+-/
+import Lean.Elab.PreDefinition.Basic
+
+namespace Lean.Elab.WF
+open Meta
+
+/--
+  Given a (dependent) tuple `t` (using `PSigma`) of the given arity.
+  Return an array containing its "elements".
+  Example: `mkTupleElems a 4` returns `#[a.1, a.2.1, a.2.2.1, a.2.2.2]`.
+  -/
+private def mkTupleElems (t : Expr) (arity : Nat) : Array Expr := do
+  let mut result := #[]
+  let mut t := t
+  for i in [:arity - 1] do
+    result := result.push (mkProj ``PSigma 0 t)
+    t := mkProj ``PSigma 1 t
+  result.push t
+
+/-- Create a unary application by packing the given arguments using `PSigma.mk` -/
+private partial def mkUnaryApp (unaryFn : Expr) (args : Array Expr) : MetaM Expr := do
+  let Expr.forallE _ d .. ← inferType unaryFn | unreachable!
+  go 0 d
+where
+  go (i : Nat) (type : Expr) : MetaM Expr := do
+    if i < args.size - 1 then
+      let arg := args[i]
+      assert! type.isAppOfArity ``PSigma 2
+      let us := type.getAppFn.constLevels!
+      let α := type.appFn!.appArg!
+      let β := type.appArg!
+      assert! β.isLambda
+      let type := β.bindingBody!.instantiate1 arg
+      let rest ← go (i+1) type
+      return mkApp4 (mkConst ``PSigma.mk us) α β arg rest
+    else
+      return args[i]
+
+/--
+  Convert the given pre-definitions into unary functions.
+  We "pack" the arguments using `PSigma`.
+-/
+def packDomain (preDefs : Array PreDefinition) : MetaM (Array PreDefinition) := do
+  let mut preDefsNew := #[]
+  let mut arities := #[]
+  let mut modified := false
+  for preDef in preDefs do
+    let (preDefNew, arity, modifiedCurr) ← lambdaTelescope preDef.value fun xs body => do
+      if xs.size > 1 then
+        let bodyType ← instantiateForall preDef.type xs
+        let mut d ← inferType xs.back
+        for x in xs.pop.reverse do
+          d ← mkAppOptM ``PSigma #[some (← inferType x), some (← mkLambdaFVars #[x] d)]
+        withLocalDeclD (← mkFreshUserName `_x) d fun tuple => do
+          let elems := mkTupleElems tuple xs.size
+          let codomain := bodyType.replaceFVars xs elems
+          let preDefNew:= { preDef with
+            declName := preDef.declName ++ `_unary
+            type := (← mkForallFVars #[tuple] codomain)
+          }
+          addAsAxiom preDefNew
+          return (preDefNew, xs.size, true)
+      else
+        return (preDef, 1, false)
+    modified := modified || modifiedCurr
+    arities := arities.push arity
+    preDefsNew := preDefsNew.push preDefNew
+  if !modified then
+    return preDefs
+  /- Return `some i` if `e` is an `preDefs[i]` application with `arities[i]` arguments.  -/
+  let isTargetApp? (e : Expr) : OptionM Nat := do
+    guard e.isApp
+    let f := e.getAppFn
+    guard f.isConst
+    let i ← preDefs.findIdx? (·.declName == f.constName!)
+    guard (e.getAppNumArgs == arities[i])
+    return i
+  -- Update values
+  for i in [:preDefs.size] do
+    let preDef := preDefs[i]
+    let preDefNew := preDefsNew[i]
+    let arity := arities[i]
+    let valueNew ← lambdaTelescope preDef.value fun xs body => do
+      if arity > 1 then
+        forallBoundedTelescope preDefNew.type (some 1) fun y _ => do
+          let newBody := body.replaceFVars xs (mkTupleElems y[0] xs.size)
+          trace[Elab.definition.wf] "newBody: {newBody}"
+          let visit (e : Expr) : MetaM TransformStep := do
+            if let some idx := isTargetApp? e |>.run then
+              let f := e.getAppFn
+              let fNew := mkConst preDefNew.declName f.constLevels!
+              let argNew ← mkUnaryApp fNew e.getAppArgs
+              return TransformStep.done <| mkApp fNew argNew
+            else
+              return TransformStep.done e
+          mkLambdaFVars y (← transform newBody (post := visit))
+      else
+        preDef.value
+    preDefsNew := preDefsNew.set! i { preDefNew with value := valueNew }
+  return preDefsNew
+
+end Lean.Elab.WF