Cite the PostScript Language Reference by its public URL

crates/eps/src/import/interp.rs

@@ -1418,8 +1418,9 @@ fn index(k: &Obj) -> Res<usize> {
     }
 }
 
-/// `known`: a key may be any object but null (PLRM 3rd ed.), and one that can't be a key here (an
-/// array, a dictionary, a real with a fraction) is in no dictionary, since `put` refuses it.
+/// `known`: a key may be any object but null (PLRM 3rd ed., §3.3.9, <https://www.adobe.com/jp/print/postscript/pdfs/PLRM.pdf>),
+/// and one that can't be a key here (an array, a dictionary, a real with a fraction) is in no
+/// dictionary, since `put` here refuses it.
 fn known(d: &DictRef, k: &Obj) -> Res<bool> {
     match k.key() {
         Some(key) => Ok(d.borrow().contains_key(&key)),

crates/eps/src/import/tests.rs

@@ -996,7 +996,8 @@ fn the_standard_dictionaries_are_dictionaries_in_systemdict() {
     check("SharedFontDirectory type /dicttype eq");
 }
 
-/// A key may be any object but null (PLRM 3rd ed.), so `known` answers for every other key.
+/// A key may be any object but null (PLRM 3rd ed., §3.3.9, <https://www.adobe.com/jp/print/postscript/pdfs/PLRM.pdf>),
+/// so `known` answers for every other key.
 /// Ghostscript's `eps2write` looks an inline image's filter up in a dictionary of abbreviations,
 /// and the filter may be an array of them.
 #[test]