Add implementations to calculate the new fields for heading.

src/types/document.rs

@@ -94,11 +94,39 @@ impl<'s> StandardProperties<'s> for Heading<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        let first_child = self.children.first();
+        let last_child = self.children.last();
+        match (first_child, last_child) {
+            (None, None) => None,
+            (None, Some(_)) | (Some(_), None) => unreachable!(),
+            (Some(first_child), Some(last_child)) => {
+                let first_child_offset = first_child.get_source().as_ptr() as usize;
+                let last_child_offset = {
+                    let last_child_source = last_child.get_source();
+                    last_child_source.as_ptr() as usize + last_child_source.len()
+                };
+                let source_offset = self.source.as_ptr() as usize;
+                debug_assert!(super::lesser_element::is_slice_of(
+                    self.source,
+                    first_child.get_source()
+                ));
+                debug_assert!(super::lesser_element::is_slice_of(
+                    self.source,
+                    last_child.get_source()
+                ));
+                Some(
+                    &self.source[(first_child_offset - source_offset)
+                        ..(last_child_offset - first_child_offset)],
+                )
+            }
+        }
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        self.children
+            .last()
+            .map(|child| child.get_post_blank())
+            .unwrap_or(0)
     }
 }
 

src/types/lesser_element.rs

@@ -610,7 +610,7 @@ fn content_line<'s>(input: &'s str) -> Res<&'s str, (Option<&'s str>, &'s str)>
 
 #[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]
 /// Check if the child string slice is a slice of the parent string slice.
-fn is_slice_of(parent: &str, child: &str) -> bool {
+pub(crate) fn is_slice_of(parent: &str, child: &str) -> bool {
     let parent_start = parent.as_ptr() as usize;
     let parent_end = parent_start + parent.len();
     let child_start = child.as_ptr() as usize;