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diff --git a/lec07/Buffer.hs b/lec07/Buffer.hs
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+module Buffer where
+
+-- Type class for data structures that can represent the text buffer
+-- of an editor.
+
+class Buffer b where
+
+  -- | Convert a buffer to a String.
+  toString :: b -> String
+
+  -- | Create a buffer from a String.
+  fromString :: String -> b
+
+  -- | Extract the nth line (0-indexed) from a buffer.  Return Nothing
+  -- for out-of-bounds indices.
+  line :: Int -> b -> Maybe String
+
+  -- | @replaceLine n ln buf@ returns a modified version of @buf@,
+  --   with the @n@th line replaced by @ln@.  If the index is
+  --   out-of-bounds, the buffer should be returned unmodified.
+  replaceLine :: Int -> String -> b -> b
+
+  -- | Compute the number of lines in the buffer.
+  numLines :: b -> Int
+
+  -- | Compute the value of the buffer, i.e. the amount someone would
+  --   be paid for publishing the contents of the buffer.
+  value :: b -> Int