Update math

public_domain.md

@@ -34,6 +34,8 @@ We therefore devise the term **safe/strong public domain**. Under this we includ
 
 If you want to create a PD work (which you should), then generally in that work **you must not use any non-public domain work**. So, for example, you can NOT create a public domain fan fiction story about Harry Potter because Harry Potter and his universe is copyrighted. Similarly you can't just use randomly googled images in a game you created because the images are most likely copyrighted. Small and obscure exceptions (fonts, freedom of panorama, ...) to this may exist in laws but it's never good to rely on them, it's best to keep it safe and simply avoid utilizing anything non-PD within your works.
 
+Also you can NOT rely on [fair use](fair_use.md)! Even though you could lawfully use someone else's copyrighted work under fair use, inclusion of such material would, by the fair use rules, limit what other would be able to do with your work, making it restricted and therefore not public domain.
+
 So you can only use your own original creations and other public domain works within your PD work. Here you should highly prefer your own creations because that is legally the safest, no one can ever challenge your right to reuse your own creation, but there is a low but considerable chance that someone else's PD work isn't actually PD or will seize to be PD by some retroactive law change. So when it only takes a small effort to e.g. photograph your own textures for a game instead of using someone else's PD textures, choose to use your own.
 
 { NOTE: The above is kind of arguing for reinventing wheels which goes a little bit against our philosophy or remixing and information sharing, but we are forced to do this by the system. We are forced to reinvent wheel to ensure that users of our works can't be legally bullied. ~drummyfish }