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+----
+-- Incoming side-effect adapter, polled by the main event loop to pump
+-- events into the dataflow graph.
+--
+-- @classmod IO
+
+class IO
+--- IO interface.
+--
+-- methods that have to be implemented by `IO` implementations.
+-- @section interface
+
+  --- construct a new instance.
+  --
+  -- Must prepare the instance for `\dirty` to be called.
+  new: =>
+
+  --- poll for changes.
+  --
+  -- Called every frame by the main event loop to update internal state.
+  tick: =>
+
+  --- check whether this adapter requires processing.
+  --
+  -- Must return a boolean indicating whether `Op`s that refer to this instance
+  -- via `Input``.io` should be notified (via `Op``\tick`). May be called multiple
+  -- times. May be called before `\tick` on the first frame after construction.
+  --
+  -- @treturn bool whether processing is required
+  dirty: =>
+
+{
+  :IO
+}