correct printing of lists when double_quotes is set to atom

src/prolog/string_list.rs

@@ -64,6 +64,11 @@ impl StringList {
         }
     }
 
+    #[inline]
+    pub fn cursor(&self) -> usize {
+        self.cursor
+    }        
+    
     #[inline]
     pub fn head(&self) -> Option<char> {
         self.borrow()[self.cursor ..].chars().next()