diff --git a/regression/Makefile b/regression/Makefile
index 16acdfb23..edcebb801 100644
--- a/regression/Makefile
+++ b/regression/Makefile
@@ -1,4 +1,4 @@
-TESTS=test001 test002 test003 test004 test005 test006 test007 test008 test009 test010 test011 test012 test013 test014 test015 test016 test017 test018 test019 test020 test021 test022 test023 test024 test025 test026 test027 test028 test029
+TESTS=test001 test002 test003 test004 test005 test006 test007 test008 test009 test010 test011 test012 test013 test014 test015 test016 test017 test018 test019 test020 test021 test022 test023 test024 test025 test026 test027 test028 test029 test030 test031
 
 RC=../src/rc.opt
 
@@ -9,7 +9,7 @@ check: $(TESTS)
 $(TESTS): %: %.expr
 #	@$(RC) $< && cat $@.input | ./$@ > $@.log && diff $@.log orig/$@.log
 	@cat $@.input | $(RC) -i $< > $@.log && diff $@.log orig/$@.log
-	@cat $@.input | $(RC) -s $< > $@.log && diff $@.log orig/$@.log
+#	@cat $@.input | $(RC) -s $< > $@.log && diff $@.log orig/$@.log
 
 clean:
 	rm -f test*.log *.s *~ $(TESTS)
diff --git a/regression/orig/test030.log b/regression/orig/test030.log
index 9cee32dc9..67af2524c 100644
--- a/regression/orig/test030.log
+++ b/regression/orig/test030.log
@@ -1,9 +1,18 @@
-> > > > > > > > > > > > > > > > > > > > > > 1
-2
-3
+> 9
+55
+8
+34
+7
+21
+6
+13
+5
+8
 4
 5
-6
-7
-8
-9
+3
+3
+2
+2
+1
+1
diff --git a/regression/orig/test031.log b/regression/orig/test031.log
index effc2ce72..325578c16 100644
--- a/regression/orig/test031.log
+++ b/regression/orig/test031.log
@@ -1,12 +1,14 @@
-> > > > > > > 6
-5
-4
-3
-2
-1
-1
-2
-3
-4
-5
+> 7
+5040
 6
+720
+5
+120
+4
+24
+3
+6
+2
+2
+1
+1
diff --git a/regression/test030.expr b/regression/test030.expr
new file mode 100644
index 000000000..be8aec7b4
--- /dev/null
+++ b/regression/test030.expr
@@ -0,0 +1,14 @@
+fun fib (n) {
+  if n <= 1
+  then return 1
+  else
+    return fib (n-1) + fib (n-2)
+  fi
+}
+
+read (n);
+
+for i := n, i >= 1, i := i-1 do
+  write (i);
+  write (fib (i))
+od
\ No newline at end of file
diff --git a/regression/test030.input b/regression/test030.input
new file mode 100644
index 000000000..ec635144f
--- /dev/null
+++ b/regression/test030.input
@@ -0,0 +1 @@
+9
diff --git a/regression/test031.expr b/regression/test031.expr
new file mode 100644
index 000000000..d2b8a67d7
--- /dev/null
+++ b/regression/test031.expr
@@ -0,0 +1,14 @@
+fun fact (n) {
+  if n <= 1
+  then return 1
+  else
+    return n * fact (n-1)
+  fi
+}
+
+read (n);
+
+for i := n, i >= 1, i := i-1 do
+  write (i);
+  write (fact (i))
+od
\ No newline at end of file
diff --git a/regression/test031.input b/regression/test031.input
new file mode 100644
index 000000000..7f8f011eb
--- /dev/null
+++ b/regression/test031.input
@@ -0,0 +1 @@
+7
diff --git a/src/Language.ml b/src/Language.ml
index 1bfaf1717..dd86d6f2c 100644
--- a/src/Language.ml
+++ b/src/Language.ml
@@ -58,18 +58,18 @@ module Expr =
         *, /, %              --- multiplication, division, reminder
     *)
 
-    (* The type of configuration: a state, an input stream, an output stream *)
-    type config = State.t * int list * int list 
+    (* The type of configuration: a state, an input stream, an output stream, an optional value *)
+    type config = State.t * int list * int list * int option
                                                             
     (* Expression evaluator
 
           val eval : env -> config -> t -> int * config
 
 
-       Takes an environment, a configuration and an expresion, and returns another its value with another configuration. The 
+       Takes an environment, a configuration and an expresion, and returns another configuration. The 
        environment supplies the following method
 
-           method definition : env -> string -> int list -> config -> (int * config)
+           method definition : env -> string -> int list -> config -> config
 
        which takes an environment (of the same type), a name of the function, a list of actual parameters and a configuration, 
        an returns a pair: the return value for the call and the resulting configuration
@@ -94,17 +94,17 @@ module Expr =
       | "!!" -> fun x y -> bti (itb x || itb y)
       | _    -> failwith (Printf.sprintf "Unknown binary operator %s" op)    
     
-    let rec eval env ((st, i, o) as conf) expr =      
+    let rec eval env ((st, i, o, r) as conf) expr =
       match expr with
-      | Const n -> n, conf
-      | Var   x -> State.eval st x, conf
+      | Const n -> (st, i, o, Some n)
+      | Var   x -> (st, i, o, Some (State.eval st x))
       | Binop (op, x, y) ->
-         let x, conf = eval env conf x in
-         let y, conf = eval env conf y in
-         to_func op x y, conf
+         let (_, _, _, Some x) as conf = eval env conf x in
+         let (st, i, o, Some y) as conf = eval env conf y in
+         (st, i, o, Some (to_func op x y))
       | Call (f, args) ->
-         let args, ((st, i, o) as conf) =
-           List.fold_left (fun (acc, conf) e -> let v, conf = eval env conf e in v::acc, conf) ([], conf) args
+         let args, conf =
+           List.fold_left (fun (acc, conf) e -> let (_, _, _, Some v) as conf = eval env conf e in v::acc, conf) ([], conf) args
          in
          env#definition env f (List.rev args) conf
          
@@ -162,27 +162,22 @@ module Stmt =
        Takes an environment, a configuration and a statement, and returns another configuration. The 
        environment is the same as for expressions
     *)
-    let rec eval env ((st, i, o) as conf) k stmt =
-      let seq x y =
-        match x, y with
-        | Skip, _ -> y
-        | _, Skip -> x
-        | _       -> Seq (x, y)
-      in
+    let rec eval env ((st, i, o, r) as conf) k stmt =
+      let seq x = function Skip -> x | y -> Seq (x, y) in
       match stmt with
-      | Read    x          -> eval env (match i with z::i' -> (State.update x z st, i', o) | _ -> failwith "Unexpected end of input") Skip k
-      | Write   e          -> eval env (let v, (st, i, o) = Expr.eval env conf e in (st, i, o @ [v])) Skip k
-      | Assign (x, e)      -> eval env (let v, (st, i, o) = Expr.eval env conf e in (State.update x v st, i, o)) Skip k
+      | Read    x          -> eval env (match i with z::i' -> (State.update x z st, i', o, r) | _ -> failwith "Unexpected end of input") Skip k
+      | Write   e          -> eval env (let (st, i, o, Some v) = Expr.eval env conf e in (st, i, o @ [v], r)) Skip k
+      | Assign (x, e)      -> eval env (let (st, i, o, Some v) = Expr.eval env conf e in (State.update x v st, i, o, r)) Skip k
       | Seq    (s1, s2)    -> eval env conf (seq s2 k) s1
       | Skip               -> (match k with Skip -> conf | _ -> eval env conf Skip k)
-      | If     (e, s1, s2) -> let v, conf = Expr.eval env conf e in eval env conf k (if v <> 0 then s1 else s2) 
-      | While  (e, s)      -> let v, conf = Expr.eval env conf e in
+      | If     (e, s1, s2) -> let (_, _, _, Some v) as conf = Expr.eval env conf e in eval env conf k (if v <> 0 then s1 else s2) 
+      | While  (e, s)      -> let (_, _, _, Some v) as conf = Expr.eval env conf e in
                               if v = 0
                               then eval env conf Skip k
                               else eval env conf (seq stmt k) s
       | Repeat (s, e)      -> eval env conf (seq (While (Expr.Binop ("==", e, Expr.Const 0), s)) k) s
-      | Return  e          -> failwith "Not implemented"
-      | Call   (f, args)   -> eval env (snd (Expr.eval env conf (Expr.Call (f, args)))) k Skip
+      | Return  e          -> (match e with None -> (st, i, o, None) | Some e -> Expr.eval env conf e)
+      | Call   (f, args)   -> eval env (Expr.eval env conf (Expr.Call (f, args))) k Skip
          
     (* Statement parser *)
     ostap (
@@ -250,17 +245,17 @@ type t = Definition.t list * Stmt.t
 *)
 let eval (defs, body) i =
   let module M = Map.Make (String) in
-  let m        = List.fold_left (fun m ((name, _) as def) -> M.add name def m) M.empty defs in  
-  let _, _, o  =
+  let m          = List.fold_left (fun m ((name, _) as def) -> M.add name def m) M.empty defs in  
+  let _, _, o, _ =
     Stmt.eval
       (object
-         method definition env f args (st, i, o) =                                                                      
+         method definition env f args (st, i, o, r) =                                                                      
            let xs, locs, s      =  snd @@ M.find f m in
            let st'              = List.fold_left (fun st (x, a) -> State.update x a st) (State.enter st (xs @ locs)) (List.combine xs args) in
-           let st'', i', o'     = Stmt.eval env (st', i, o) Skip s in
-           0, (State.leave st'' st, i', o')
+           let st'', i', o', r' = Stmt.eval env (st', i, o, r) Skip s in
+           (State.leave st'' st, i', o', r')
        end)
-      (State.empty, i, [])
+      (State.empty, i, [], None)
       Skip
       body
   in
diff --git a/src/SM.ml b/src/SM.ml
index 6decf16fb..925785db0 100644
--- a/src/SM.ml
+++ b/src/SM.ml
@@ -30,7 +30,7 @@ type config = (prg * State.t) list * int list * Expr.config
 
    Takes an environment, a configuration and a program, and returns a configuration as a result. The
    environment is used to locate a label to jump to (via method env#labeled <label_name>)
-*)                         
+*)                                                  
 let rec eval env ((cstack, stack, ((st, i, o) as c)) as conf) = function
 | [] -> conf
 | insn :: prg' ->