Add a DocumentRoot context element storing the original full document.

This might be used for look-behind instead of storing previous element nodes in the context tree.

src/parser/document.rs

@@ -2,6 +2,7 @@
 use super::combinator::context_many1;
 use super::error::Res;
 use super::paragraph::paragraph;
+use super::parser_context::ContextElement;
 use super::parser_context::ContextTree;
 use super::token::Paragraph;
 use super::token::Token;
@@ -14,7 +15,9 @@ type UnboundMatcher<'r, 's, I, O, E> = dyn Fn(Context<'r, 's>, I) -> IResult<I,
 
 pub fn document(input: &str) -> Res<&str, Vec<Paragraph>> {
     let initial_context: ContextTree<'_, '_> = ContextTree::new();
-    let (remaining, tokens) = context_many1(&initial_context, paragraph)(input)?;
+    let document_context =
+        initial_context.with_additional_node(ContextElement::DocumentRoot(input));
+    let (remaining, tokens) = context_many1(&document_context, paragraph)(input)?;
     let paragraphs = tokens
         .into_iter()
         .map(|token| match token {