diff --git a/Setup.hs b/Setup.hs
--- a/Setup.hs
+++ b/Setup.hs
@@ -1,2 +1,6 @@
-import Distribution.Simple
-main = defaultMain
+import Distribution.Simple (defaultMainWithHooks)
+import Distribution.Simple.UUAGC (uuagcLibUserHook)
+import UU.UUAGC(uuagc)
+
+main :: IO ()
+main = defaultMainWithHooks (uuagcLibUserHook uuagc)
diff --git a/dist/build/ewe/ewe-tmp/Language/EWE/Parser.hs b/dist/build/ewe/ewe-tmp/Language/EWE/Parser.hs
--- a/dist/build/ewe/ewe-tmp/Language/EWE/Parser.hs
+++ b/dist/build/ewe/ewe-tmp/Language/EWE/Parser.hs
@@ -74,7 +74,7 @@
 happyGotoOffsets = HappyA# "\x02\x00\x2c\x00\x45\x00\x27\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x43\x00\x00\x00\x00\x00\x00\x00\x32\x00\x00\x00\x00\x00\x00\x00\x00\x00\x42\x00\x41\x00\x40\x00\x3f\x00\x3e\x00\x3d\x00\x30\x00\x3c\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x3b\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x3a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x39\x00\x00\x00\x38\x00\x37\x00\x35\x00\x2b\x00\x26\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0f\x00\x00\x00\x04\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"#
 
 happyDefActions :: HappyAddr
-happyDefActions = HappyA# "\x00\x00\x00\x00\xe3\xff\xfd\xff\xfa\xff\x00\x00\xe0\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xe5\xff\xe4\xff\x00\x00\x00\x00\xe0\xff\x00\x00\xe9\xff\xe8\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xfc\xff\xfe\xff\x00\x00\x00\x00\xf5\xff\xf9\xff\xf8\xff\x00\x00\x00\x00\xfb\xff\xf6\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xda\xff\xdb\xff\xdc\xff\xdd\xff\xde\xff\xdf\xff\x00\x00\xea\xff\x00\x00\xec\xff\xed\xff\xe1\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xf0\xff\xf1\xff\xf2\xff\xf3\xff\xf4\xff\xf7\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xeb\xff\x00\x00\x00\x00\x00\x00\xe3\xff\x00\x00\x00\x00\xe7\xff\xe6\xff\xee\xff\xef\xff\xe2\xff"#
+happyDefActions = HappyA# "\x00\x00\x00\x00\xe3\xff\xfd\xff\xfa\xff\x00\x00\xe0\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xe5\xff\xe4\xff\x00\x00\x00\x00\xe0\xff\x00\x00\xe9\xff\xe8\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xfc\xff\xfe\xff\x00\x00\x00\x00\xf5\xff\xf9\xff\xf8\xff\x00\x00\x00\x00\xfb\xff\xf6\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xda\xff\xdb\xff\xde\xff\xdf\xff\xdc\xff\xdd\xff\x00\x00\xea\xff\x00\x00\xec\xff\xed\xff\xe1\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xf0\xff\xf1\xff\xf2\xff\xf3\xff\xf4\xff\xf7\xff\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xeb\xff\x00\x00\x00\x00\x00\x00\xe3\xff\x00\x00\x00\x00\xe7\xff\xe6\xff\xee\xff\xef\xff\xe2\xff"#
 
 happyCheck :: HappyAddr
 happyCheck = HappyA# "\xff\xff\x03\x00\x00\x00\x01\x00\x02\x00\x03\x00\x01\x00\x05\x00\x03\x00\x05\x00\x01\x00\x02\x00\x03\x00\x03\x00\x0b\x00\x0c\x00\x0d\x00\x0e\x00\x0f\x00\x04\x00\x16\x00\x17\x00\x18\x00\x19\x00\x1a\x00\x1b\x00\x1c\x00\x1d\x00\x17\x00\x1f\x00\x20\x00\x16\x00\x17\x00\x17\x00\x10\x00\x11\x00\x12\x00\x13\x00\x14\x00\x15\x00\x01\x00\x02\x00\x03\x00\x05\x00\x05\x00\x01\x00\x02\x00\x03\x00\x05\x00\x05\x00\x02\x00\x03\x00\x01\x00\x05\x00\x03\x00\x01\x00\x06\x00\x03\x00\x05\x00\x21\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x05\x00\x04\x00\x01\x00\x04\x00\x1e\x00\x09\x00\x01\x00\x09\x00\x1c\x00\x07\x00\x01\x00\x0b\x00\x09\x00\x01\x00\x08\x00\x01\x00\x09\x00\x07\x00\x0a\x00\x08\x00\x07\x00\xff\xff\x07\x00\x03\x00\xff\xff\x06\x00\x04\x00\x0b\x00\x0b\x00\x08\x00\x06\x00\x06\x00\x06\x00\x05\x00\x04\x00\x08\x00\xff\xff\xff\xff\x22\x00\xff\xff\xff\xff\x17\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x21\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"#
diff --git a/ewe.cabal b/ewe.cabal
--- a/ewe.cabal
+++ b/ewe.cabal
@@ -2,8 +2,8 @@
 -- see http://haskell.org/cabal/users-guide/
 
 name:                ewe
-version:             0.1.0.23
-synopsis:            An language to teach a programming
+version:             0.1.0.30
+synopsis:            An language using in Programming Languages teaching
 description:         Another implemention of the EWE programming language originally created and developed by Kent D. Lee. EWE is an extension of the RAM programming language. RAM was created by Sethi.
 homepage:            http://github.com/jfcmacro/ewe
 license:             BSD3
@@ -20,14 +20,19 @@
   Build-depends:       base >=4.5 && < 5,
                        transformers >=0.3,
                        mtl >= 2.1,
+                       containers >= 0.4.2,
                        array >= 0.4.0.0
   hs-source-dirs:      src
   build-tools:         alex, happy
   other-modules:       Language.EWE.AbsSyn,
+                       Language.EWE.Utils,
                        Language.EWE.VM,
                        Language.EWE.Token,
                        Language.EWE.Scanner,
+                       Language.EWE.CheckGrammar,
                        Language.EWE.Parser
+  x-agmodule:          file: "src/Language/EWE/AbsSyn.ag" options : data, pretty, haskellsyntax, module "Language.EWE.AbsSyn"
+  x-agmodule:          file: "src/Language/EWE/CheckGrammar.ag" options : semfuns, pretty, haskellsyntax, catas, wrappers, signatures, module "Language.EWE.CheckGrammar"
 
 source-repository head
   type:	    git
diff --git a/src/Language/EWE/AbsSyn.ag b/src/Language/EWE/AbsSyn.ag
new file mode 100644
--- /dev/null
+++ b/src/Language/EWE/AbsSyn.ag
@@ -0,0 +1,57 @@
+
+data MRef | MRefI  Int
+          | MRefId String
+
+deriving MRef : Eq, Show
+
+data Cond | CLET
+          | CLT
+          | CGET
+          | CGT
+          | CE
+          | CNE
+
+deriving Cond: Eq, Show
+
+type Equ = (String,Int)
+
+type Equates = [Equ]
+
+type Labels = [String]
+
+data Stmt | Stmt labels :: Labels instr :: Instr
+
+deriving Stmt: Eq, Show
+
+type Stmts = [Stmt]
+
+data Prog | Prg stms    :: Stmts
+                equates :: Equates
+
+deriving Prog : Eq, Show
+
+data Instr | IMMI  dest :: MRef Int         -- Move a int into memory
+           | IMMS  dest :: MRef String      -- Move a string into memory
+           | IMRPC dest :: MRef Int         -- Move a relative PC into memory
+           | SPC   oper :: MRef             -- Set PC from memory
+           | IMMM  dest,oper :: MRef        -- Move Memory to Memory
+           | IAdd  dest,left,right :: MRef  -- Add two Memory References
+           | ISub  dest,left,right :: MRef  -- Sub two Memory References
+           | IMul  dest,left,right :: MRef  -- Mul two Memory References
+           | IDiv  dest,left,right :: MRef  -- Div two Memory References
+           | IMod  dest,left,right ::  MRef -- Mod two Memory References
+           | IMRI  dest,oper :: MRef Int    -- Move to Memory Ref
+           | IMMR  dest :: MRef Int oper :: MRef -- Move to Memory Ref
+           | IRI   oper :: MRef             -- Read an Int into a Mem Ref
+           | IWI   oper :: MRef             -- Write an Int from a Mem Ref
+           | IRS   left, right :: MRef      -- Read a Str
+           | IWS   oper :: MRef             -- Write a Str
+           | IGI  Int                       -- Goto to line
+           | IGS  String                    -- Goto sym
+           | IFI  bcond :: MRef Cond then :: MRef Int        -- If Cond then Int
+           | IFS  bcond :: MRef Cond then :: MRef String     --
+           | IH                             -- Halt
+           | IB                             -- Break
+           | INI
+
+deriving Instr : Eq, Show
diff --git a/src/Language/EWE/AbsSyn.hs b/src/Language/EWE/AbsSyn.hs
deleted file mode 100644
--- a/src/Language/EWE/AbsSyn.hs
+++ /dev/null
@@ -1,69 +0,0 @@
-module Language.EWE.AbsSyn(MRef(..)
-                          ,Cond(..)
-                          ,Equ
-                          ,Equates
-                          ,Labels
-                          ,Stmt(..)
-                          ,Stmts
-                          ,Prog(..)
-                          ,Instr(..)
-                          ,emptyProg) where
-
-data AbsSyn = Empty
-
-data MRef = MRefI  Int
-          | MRefId String
-          deriving (Eq, Show)
-
-data Cond = CLET
-          | CLT
-          | CGET
-          | CGT
-          | CE
-          | CNE
-          deriving (Eq,Show)
-
-type Equ = (String,Int)
-
-type Equates = [Equ]
-
-type Labels = [String]
-
-data Stmt = Stmt Labels Instr
-            deriving (Eq,Show)
-
-type Stmts = [Stmt]
-
-data Prog = Prg {stms    :: Stmts
-                ,equates :: Equates}
-            deriving (Eq,Show)
-
-data Instr = IMMI MRef Int       -- Move Memory Int
-           | IMMS MRef String    -- Move Memory String
-           | IMRPC MRef Int      -- Move Memory Relative PC
-           | SPC  MRef           -- Set PC
-           | IMMM MRef MRef      -- Move Memory to Memory
-           | IAdd MRef MRef MRef -- Add two Memory References
-           | ISub MRef MRef MRef -- Sub two Memory References
-           | IMul MRef MRef MRef -- Mul two Memory References
-           | IDiv MRef MRef MRef -- Div two Memory References
-           | IMod MRef MRef MRef -- Mod two Memory References
-           | IMRI MRef MRef Int  -- Move to Memory ref a Indexed Memory Pos
-           | IMMR MRef Int  MRef -- Move to Index Memory Ref a Memory Ref
-           | IRI  MRef           -- Read an Int and stored into a Mem Ref
-           | IWI  MRef           -- Write an Int from a Mem Ref
-           | IRS  MRef MRef      -- Read a Str
-           | IWS  MRef           -- Write a Str
-           | IGI  Int            -- Goto to line
-           | IGS  String         -- Goto sym
-           | IFI  MRef Cond MRef Int -- If Cond then Int
-           | IFS  MRef Cond MRef String --
-           | IH                  -- Halt
-           | IB                  -- Break
-           | INI
-            deriving (Eq,Show)
-
-emptyProg :: Prog
-emptyProg = Prg { stms = []
-                , equates = []
-                }
diff --git a/src/Language/EWE/CheckGrammar.ag b/src/Language/EWE/CheckGrammar.ag
new file mode 100644
--- /dev/null
+++ b/src/Language/EWE/CheckGrammar.ag
@@ -0,0 +1,124 @@
+imports
+{
+import qualified Data.Set as Set
+import qualified Data.Map as Map
+import Language.EWE.AbsSyn
+import Language.EWE.Utils
+}
+
+attr Prog
+   syn res      :: {Either String ()}
+
+attr Equates
+   chn idMemRef :: {Map.Map String Int}
+
+attr Equ
+   chn idMemRef :: {Map.Map String Int}
+   syn res      :: {Either String ()}
+
+attr Stmts
+   chn labelDef                     :: {Set.Set String}
+   inh labelFound                   :: {Set.Set String}
+   inh idMemRefDef                  :: {Map.Map String Int}
+   syn res use {(<-->)} {Right ()} :: {Either String ()}
+
+attr Stmt
+   chn labelDef    :: {Set.Set String}
+   inh labelFound  :: {Set.Set String}
+   inh idMemRefDef :: {Map.Map String Int}
+   syn res         :: {Either String ()}
+
+attr Instr
+   inh labelFound  :: {Set.Set String}
+   inh idMemRefDef :: {Map.Map String Int}
+   syn res         :: {Either String ()}
+
+attr Labels
+   syn res      :: { Either String () }
+   chn labelDef :: { Set.Set String }
+   syn copy     :: self
+
+attr MRef
+   inh idMemRefDef :: { Map.Map String Int }
+   syn res         :: { Either String () }
+
+sem Equ
+  | Tuple lhs.res      = { if @x1 `Map.member` @lhs.idMemRef
+                           then Left $ @x1 ++ " was already defined "
+                           else Right ()
+                         }
+             .idMemRef = { Map.insert @x1 @x2 @lhs.idMemRef }
+
+sem Labels
+  | Cons  lhs.res      = { isLabelNotDef @hd @tl.labelDef }
+             .labelDef = { Set.insert @hd @tl.labelDef }
+  | Nil   lhs.res      = { Right ()  }
+             .labelDef = { @lhs.labelDef }
+
+sem Prog
+  | Prg stms.labelDef    = { Set.empty }
+            .labelFound  = { @stms.labelDef }
+            .idMemRefDef = { @equates.idMemRef }
+        equates.idMemRef = { Map.empty }
+
+sem Stmt
+  | Stmt loc.res1 = {
+   let f (e,er,s) lbl = if Set.member lbl s
+                        then (True,
+                              lbl ++ " is already defined\n" ++ er,
+                              s)
+                        else (e, er, Set.insert lbl s)
+       (e', errs', set') = foldl f (False,"",@lhs.labelDef) @labels.copy
+   in if e'
+      then Left errs'
+      else Right ()
+                   }
+          lhs.res = { @res1 <--> @instr.res }
+
+sem MRef
+  | MRefI  lhs.res = Right ()
+  | MRefId lhs.res = isMemRefDef @string @lhs.idMemRefDef
+
+sem Instr
+  | IMMI  lhs.res = { @dest.res }
+  | IMMS  lhs.res = { @dest.res }
+  | IMRPC lhs.res = { @dest.res }
+  | SPC   lhs.res = { @oper.res }
+  | IMMM  lhs.res = { @dest.res <--> @oper.res }
+  | IAdd  lhs.res = { @dest.res <--> @left.res <--> @right.res }
+  | ISub  lhs.res = { @dest.res <--> @left.res <--> @right.res }
+  | IMul  lhs.res = { @dest.res <--> @left.res <--> @right.res }
+  | IDiv  lhs.res = { @dest.res <--> @left.res <--> @right.res }
+  | IMod  lhs.res = { @dest.res <--> @left.res <--> @right.res }
+  | IMRI  lhs.res = { @dest.res <--> @oper.res }
+  | IMMR  lhs.res = { @dest.res <--> @oper.res }
+  | IRI   lhs.res = { @oper.res }
+  | IWI   lhs.res = { @oper.res }
+  | IRS   lhs.res = { @left.res <--> @right.res }
+  | IWS   lhs.res = { @oper.res }
+  | IGI   lhs.res = { Right () }
+  | IGS   lhs.res = { isLabelDef @string @lhs.labelFound }
+  | IFI   lhs.res = { @bcond.res <--> @then.res }
+  | IFS   lhs.res = { isLabelDef @string @lhs.labelFound <--> @bcond.res <--> @then.res}
+  | IH    lhs.res = { Right () }
+  | IB    lhs.res = { Right () }
+  | INI   lhs.res = { Right () }
+
+include "AbsSyn.ag"
+
+{
+isLabelDef :: String -> Set.Set String ->  Either String ()
+isLabelDef str sets
+    | str `Set.member` sets = Right ()
+    | otherwise             = Left $ "Label: " ++ str ++ " is not defined "
+
+isLabelNotDef :: String -> Set.Set String ->  Either String ()
+isLabelNotDef str sets
+    | str `Set.member` sets =  Left $ "Label: " ++ str ++ " is already defined "
+    | otherwise             = Right ()
+
+isMemRefDef :: String -> Map.Map String Int -> Either String ()
+isMemRefDef str map
+    | str `Map.member` map = Right ()
+    | otherwise            = Left $ "Mem Ref: " ++ str ++ " is not defined"
+}
diff --git a/src/Language/EWE/Parser.y b/src/Language/EWE/Parser.y
--- a/src/Language/EWE/Parser.y
+++ b/src/Language/EWE/Parser.y
@@ -86,10 +86,10 @@
 MemRef : 'M' '[' int ']'                          { MRefI $3 }
        | id                                       { MRefId $1 }
 
-Cond : '>='                                       { CLET }
-     | '>'                                        { CLT }
-     | '<='                                       { CGET }
-     | '<'                                        { CGT }
+Cond : '<='                                       { CLET }
+     | '<'                                        { CLT }
+     | '>='                                       { CGET }
+     | '>'                                        { CGT }
      | '='                                        { CE }
      | '<>'                                       { CNE }
 
diff --git a/src/Language/EWE/Utils.hs b/src/Language/EWE/Utils.hs
new file mode 100644
--- /dev/null
+++ b/src/Language/EWE/Utils.hs
@@ -0,0 +1,27 @@
+module Language.EWE.Utils(emptyProg,
+                          equates,
+                          stms,
+                          (<-->),
+                          isMemRefId) where
+
+import Language.EWE.AbsSyn
+
+emptyProg :: Prog
+emptyProg = Prg [] []
+
+equates :: Prog -> Equates
+equates (Prg _ e) = e
+
+stms :: Prog -> Stmts
+stms (Prg s _) = s
+
+infixl 5 <-->
+
+(<-->) :: Either String () -> Either String () -> Either String ()
+(Left s) <--> (Left s') = Left (s ++ "\n" ++ s')
+(Left s) <--> _         = Left s
+_       <--> (Left s')  = Left s'
+_       <--> a          = a
+
+isMemRefId :: MRef -> Bool
+isMemRefId (MRefId _) = True
diff --git a/src/Language/EWE/VM.hs b/src/Language/EWE/VM.hs
--- a/src/Language/EWE/VM.hs
+++ b/src/Language/EWE/VM.hs
@@ -2,6 +2,7 @@
 module Language.EWE.VM(runVM,execVM) where
 
 import Language.EWE.AbsSyn
+import Language.EWE.Utils(emptyProg, equates, stms)
 import qualified Data.List as L
 import qualified Data.Maybe as M
 import Control.Monad.Trans.State.Lazy
@@ -32,7 +33,7 @@
 
 type StateVMM = StateT StateVM IO
 
-runVM :: Prog -> IO () 
+runVM :: Prog -> IO ()
 runVM prg = evalStateT evalVM (initVM prg)
 
 execVM :: Prog -> IO StateVM
@@ -42,13 +43,13 @@
 evalVM = do
   lift $ hPutStrLn stdout "Iniciando maquina"
   evalInstr
-  
+
 initVM :: Prog -> StateVM
 initVM p = emptyStateVM { prg = p }
 
 mRef :: MRef -> Equates -> Int
 mRef (MRefI i) _  = i
-mRef (MRefId s) m = M.fromMaybe (error "No found") $ L.lookup s m
+mRef (MRefId s) m = M.fromMaybe (error $ "Internal error: memory reference " ++ s ++ " not found ") $ L.lookup s m
 
 outMem :: Int -> Memory -> (Memory,Int)
 outMem r m = M.maybe ((r,0):m, 0) (\i -> (m,i)) (L.lookup r m)
@@ -107,9 +108,12 @@
   state { mem = inMem (mRef mr (ge state)) i (mem state)
         , pc  = incrPC state
         }
-execInstr (IMMS mr s) state =
-  state { mem = inMem (mRef mr (ge state)) (mRef (MRefId s) (ge state)) (mem state)
-        , pc = incrPC state
+execInstr (IMMS mr s) st = 
+  let base  = mRef mr (ge st)
+      total = base + length s
+      st'   = moveStrInMem s base total st
+  in st { mem = mem st'
+        , pc = incrPC st
         }
 execInstr (IMRPC mr i) state =
   state { mem = inMem (mRef mr (ge state)) ((pc state) + i) (mem state)
@@ -125,9 +129,9 @@
   in state { mem = inMem (mRef mr1 ge') v m'
            , pc  = incrPC state
            }
-execInstr (IAdd mrr mra mrb) state = comp (+) mrr mra mrb state
-execInstr (ISub mrr mra mrb) state = comp (-) mrr mra mrb state
-execInstr (IMul mrr mra mrb) state = comp (*) mrr mra mrb state
+execInstr (IAdd mrr mra mrb) state = comp (+)   mrr mra mrb state
+execInstr (ISub mrr mra mrb) state = comp (-)   mrr mra mrb state
+execInstr (IMul mrr mra mrb) state = comp (*)   mrr mra mrb state
 execInstr (IDiv mrr mra mrb) state = comp (div) mrr mra mrb state
 execInstr (IMod mrr mra mrb) state = comp (mod) mrr mra mrb state
 execInstr (IMRI mrr mr  i) state =
@@ -166,7 +170,7 @@
       stms'   = stms prg'
       l'      = length stms'
       (m',a)  = outMem (mRef mra ge') m
-      (m'',b) = outMem (mRef mra ge') m'
+      (m'',b) = outMem (mRef mrb ge') m'
       op      = fun cond
       i'      = if (i >= 0 && i < l')
                 then i else error "Inst Pos not valid in condition"
@@ -182,7 +186,7 @@
       stms'   = stms prg'
       l'      = length stms'
       (m',a)  = outMem (mRef mra ge') m
-      (m'',b) = outMem (mRef mra ge') m'
+      (m'',b) = outMem (mRef mrb ge') m'
       op      = fun cond
       i       = lookupLabel s stms'
       npc     = if a `op` b then i else pc' + 1
@@ -244,8 +248,8 @@
 lookupLabel' n s [] = error "Label not found"
 lookupLabel' n s ((Stmt []  _):stms) = lookupLabel' (n+1) s stms
 lookupLabel' n s ((Stmt lbls _):stms)
-  | s `elem` lbls          = n
-  | otherwise            = lookupLabel' (n+1) s stms
+    | s `elem` lbls = n
+    | otherwise     = lookupLabel' (n+1) s stms
 
 fun :: Cond -> (Int -> Int -> Bool)
 fun CLET = (<=)
diff --git a/src/Main.hs b/src/Main.hs
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -14,6 +14,7 @@
 import Language.EWE.Parser
 import Language.EWE.AbsSyn
 import Language.EWE.VM(runVM,execVM)
+import Language.EWE.CheckGrammar
 
 data Options =  Options { optShowVersion :: Bool
                         , optNoExec      :: Bool
@@ -30,7 +31,7 @@
 
 options :: [OptDescr (Options -> Options)]
 options =
-  [ Option ['V','?'] ["version"]
+  [ Option ['v','?'] ["version"]
     (NoArg (\opts -> opts { optShowVersion  = True }))
     "show version number"
   , Option ['n'] ["noexec"]
@@ -77,12 +78,25 @@
   s    <- hGetContents fh
   let  scanout = runAlex s alexExec
        pRes   = pEWE s
-       errorParser  = either (\_ -> True) (\_ -> False) pRes
-  when (optScanOut opts) (showScannerOutput scanout)
-  when (optParserOut opts) (showParserOutput pRes)
-  when (optNoExec opts) (execProg errorParser pRes)
+       errorParser           = either (\_ -> True)  (\_ -> False) pRes
+       (passGrammar,errGram) = either (\_ -> (False,[])) checkGrammar  pRes
+  when (optScanOut opts)               (showScannerOutput scanout)
+  when (optParserOut opts)             (showParserOutput pRes)
+  when (not passGrammar)               (showErrorGrammar errGram)
+  when (optNoExec opts && passGrammar) (execProg errorParser pRes)
   hClose fh
 
+showErrorGrammar :: String -> IO ()
+showErrorGrammar msg = do
+   hPutStrLn stderr $ show msg
+
+checkGrammar :: Prog -> (Bool,String)
+checkGrammar prog = let inh = Inh_Prog
+                        syn = wrap_Prog (sem_Prog prog) inh
+                    in case (res_Syn_Prog syn) of
+                         Left msg -> (False, msg)
+                         Right () -> (True, [])
+ 
 processStaticOptions :: Options -> IO ()
 processStaticOptions opts =
   if optShowVersion opts