UI backends can either implement the above two words in the case of an asynchronous clipboard model (for example, X11). If direct access to the clipboard is provided (Windows, macOS), the following two generic words may be implemented instead:
However, gadgets should not call these words, since they will fail if only the asynchronous method of clipboard access is supported by the backend in use.
These variables may contain clipboard protocol implementations which transfer data to and from the native system clipboard. However an UI backend may leave one or both of these variables in their default state, which is a trivial clipboard implementation internal to the Factor UI.