Implement document parser.

src/parser/document.rs

@@ -5,6 +5,7 @@ use nom::character::complete::space1;
 use nom::combinator::eof;
 use nom::combinator::map;
 use nom::combinator::not;
+use nom::combinator::opt;
 use nom::combinator::recognize;
 use nom::combinator::verify;
 use nom::multi::many0;
@@ -74,8 +75,19 @@ pub fn document(input: &str) -> Res<&str, Document> {
     let initial_context: ContextTree<'_, '_> = ContextTree::new();
     let document_context =
         initial_context.with_additional_node(ContextElement::DocumentRoot(input));
-
-    todo!()
+    let section_matcher = parser_with_context!(section)(&document_context);
+    let heading_matcher = parser_with_context!(heading)(&document_context);
+    let (remaining, zeroth_section) = opt(section_matcher)(input)?;
+    let (remaining, children) = many0(heading_matcher)(remaining)?;
+    let source = get_consumed(input, remaining);
+    Ok((
+        remaining,
+        Document {
+            source,
+            zeroth_section,
+            children,
+        },
+    ))
 }
 
 fn section<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, Section<'s>> {
@@ -114,7 +126,7 @@ fn heading<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, Hea
     Ok((
         remaining,
         Heading {
-            source: source,
+            source,
             stars: star_count,
             children,
         },