Disable stack modification by branching (j*) instructions

app/VirtualMachine/Instruction.hs

@@ -110,6 +110,7 @@ jump [] _  = throwError "Address expected"
 jumpIf :: (Int -> Int -> Bool) -> Params -> Pops -> ExceptT String Machine ()
 jumpIf p (addr:_) (top:_) = lift $ do
   pc <- getPc
+  push [top]
   setPc $ if top `p` 0 then addr else pc + 2
   return ()
 jumpIf _ [] _  = throwError "Address expected"

app/VirtualMachine/VM.hs

@@ -42,7 +42,7 @@ data Op = Nop  -- 0x00
         | Out  -- 0x17
         | Clr  -- 0x18
         | Roll -- 0x19
-        | Over -- 0x20
+        | Over -- 0x1A
         deriving (Eq, Ord, Enum, Show, Read, Bounded)
 
 type Machine = StateT VM IO