Implement the new fields for latex fragment.

src/types/object.rs

@@ -194,6 +194,7 @@ pub struct Entity<'s> {
 pub struct LatexFragment<'s> {
     pub source: &'s str,
     pub value: &'s str,
+    pub post_blank: Option<&'s str>,
 }
 
 #[derive(Debug)]
@@ -759,11 +760,15 @@ impl<'s> StandardProperties<'s> for LatexFragment<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        None
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        self.post_blank
+            .map(|post_blank| post_blank.chars().count())
+            .unwrap_or(0)
+            .try_into()
+            .expect("Too much post-blank to fit into a PostBlank.")
     }
 }