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A basic technique is to use newtype declarations to declare separate 
types for separate intents.
...

     append (AnnotatedString x) (AnnotatedString y)
       = AnnotatedString (x ++ y)

   If x and y have the same safety level,
   then (x ++ y) has again that same safety level.

   type family Join a b
   type instance Join Safe Safe = Safe
   type instance Join Safe Unsafe = Unsafe
   type instance Join Unsafe Safe = Unsafe
   type instance Join Unsafe Unsafe = Unsafe

   append
     :: AnnotatedString a
     -> AnnotatedString b
     -> AnnotatedString (Join a b)

   (x ++ y) is at least as safe as the least safe of x and y.