Code background grey, use special code-block for GHCi

VL1/content/VL1_arithmetic_and_co.tex

@@ -1,9 +1,8 @@
-GHCi:
-\begin{haskellcode}
+\begin{haskellcode*}{bgcolor=mygrey,frame=single,numbers=none,label=GHCi}
 > 3 + 5
 > (3 :: Integer) + (5 :: Int)
 > 6 * 5.0
 > "Hello" ++ " world"
 > "Haskell" > "C++"
 > True && False
-\end{haskellcode}
+\end{haskellcode*}