[Haskell-beginners] Writing a custom pop function for a stack data type

[doaltan]

Hi I have such a stack data structure: 
datamystack =Empty |Elem Char mystack derivingShow

I'm trying to get the head of the stack using this:
pophead :: Stack -> Char
pophead Empty = Empty 
pophead (Element x stack) = x
And I'm getting this error for the last sentence of the function :
Not in scope: data constructor `Stack'
Can you tell me how to fix it? 
Thanks.
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