diff --git a/Cargo.toml b/Cargo.toml
index ca2804c..30ffac8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "mup"
-version = "0.1.0"
+version = "0.0.0"
 edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
diff --git a/src/parse.rs b/src/parse.rs
index f24255b..5cc4a4f 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -1,3 +1,46 @@
+pub trait ParseExtensions<'a> {
+	type Output;
+	type Error;
+
+	fn parse(
+		&self,
+		content: &'a str,
+		span: Span,
+	) -> core::result::Result<Self::Output, Self::Error>;
+}
+impl<'a, T, E, F> ParseExtensions<'a> for F
+where
+	F: Fn(&'a str, Span) -> core::result::Result<T, E>,
+{
+	type Output = T;
+
+	type Error = E;
+
+	fn parse(
+		&self,
+		content: &'a str,
+		span: Span,
+	) -> core::result::Result<Self::Output, Self::Error> {
+		self(content, span)
+	}
+}
+
+// impl<'a, T, E> ParseExtension<'a>
+// 	for &fn(&'a str, Span) -> core::result::Result<T, E>
+// {
+// 	type Output = T;
+
+// 	type Error = E;
+
+// 	fn parse(
+// 		&self,
+// 		content: &'a str,
+// 		span: Span,
+// 	) -> core::result::Result<Self::Output, Self::Error> {
+// 		self(content, span)
+// 	}
+// }
+
 #[derive(Debug, Copy, Clone)]
 struct InvalidCharBoundary;
 trait StrExt {
@@ -38,11 +81,16 @@ impl From<ByteOffset> for usize {
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub struct Span {
-	start: ByteOffset,
+	pub start: ByteOffset,
 	/// exclusive
-	end: ByteOffset,
+	pub end: ByteOffset,
 }
-fn span<A, B>(start: A, end: B) -> Span
+impl Span {
+	pub fn enclose<T>(self, t: T) -> Spanned<T> {
+		spanned(self.start, self.end, t)
+	}
+}
+pub fn span<A, B>(start: A, end: B) -> Span
 where
 	A: Into<ByteOffset>,
 	B: Into<ByteOffset>,
@@ -55,11 +103,11 @@ where
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub struct Spanned<T> {
-	span: Span,
-	value: T,
+	pub span: Span,
+	pub value: T,
 }
 
-fn spanned<A, B, T>(start: A, end: B, value: T) -> Spanned<T>
+pub fn spanned<A, B, T>(start: A, end: B, value: T) -> Spanned<T>
 where
 	A: Into<ByteOffset>,
 	B: Into<ByteOffset>,
@@ -70,14 +118,15 @@ where
 }
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub enum Error {
+pub enum Error<ExtError> {
 	UnclosedExtension,
+	Extension(ExtError),
 }
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub struct Header<S> {
-	level: u8,
-	content: S,
+	pub level: u8,
+	pub content: S,
 }
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
@@ -87,23 +136,24 @@ pub struct Text<S>(pub S);
 pub struct Extension<S>(pub S);
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub enum ParagraphPiece<S> {
-	Text(Text<S>),
-	Extension(Extension<S>),
+pub enum ParagraphPiece<StrRepr, ExtRepr> {
+	Text(Text<StrRepr>),
+	Extension(ExtRepr),
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
-pub struct Paragraph<S> {
-	pieces: Vec<Spanned<ParagraphPiece<S>>>,
+pub struct Paragraph<StrRepr, ExtRepr> {
+	pub pieces: Vec<Spanned<ParagraphPiece<StrRepr, ExtRepr>>>,
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
-pub enum Block<S> {
-	Header(Header<S>),
-	Paragraph(Paragraph<S>),
+pub enum Block<StrRepr, ExtRepr> {
+	Header(Header<StrRepr>),
+	Paragraph(Paragraph<StrRepr, ExtRepr>),
 }
 
-pub type Result<T> = core::result::Result<T, Spanned<Error>>;
+pub type Result<T, ExtError> =
+	core::result::Result<T, Spanned<Error<ExtError>>>;
 
 #[derive(Copy, Clone)]
 struct State<'a> {
@@ -175,9 +225,10 @@ impl<'a> State<'a> {
 	}
 }
 
-fn header<'a, S>(state: &mut State<'a>) -> Result<Header<S>>
+fn header<'a, S, E>(state: &mut State<'a>) -> Result<Header<S>, E::Error>
 where
 	S: From<&'a str>,
+	E: ParseExtensions<'a>,
 {
 	let mut level = 1;
 
@@ -212,9 +263,13 @@ where
 	})
 }
 
-fn extension<'a, S>(state: &mut State<'a>) -> Result<Extension<S>>
+fn extension<'a, S, E>(
+	state: &mut State<'a>,
+	pex: &E,
+) -> Result<E::Output, E::Error>
 where
 	S: From<&'a str>,
+	E: ParseExtensions<'a>,
 {
 	debug_assert!({
 		let mut peek_buf = [None; 2];
@@ -246,7 +301,9 @@ where
 
 				let content = &state.corpus[start.0..bo.0];
 
-				return Ok(Extension(content.into()));
+				return pex
+					.parse(content, span(start.0, bo.0))
+					.map_err(|e| spanned(start.0, bo.0, Error::Extension(e)));
 			}
 			_ => {
 				state.forward();
@@ -256,9 +313,10 @@ where
 }
 
 
-fn text<'a, S>(state: &mut State<'a>) -> Result<Text<S>>
+fn text<'a, S, E>(state: &mut State<'a>) -> Result<Text<S>, E::Error>
 where
 	S: From<&'a str>,
+	E: ParseExtensions<'a>,
 {
 	let start = state.current_offset;
 
@@ -280,9 +338,13 @@ where
 	Ok(Text(state.corpus[start.0..end.0].into()))
 }
 
-fn paragraph<'a, S>(state: &mut State<'a>) -> Result<Paragraph<S>>
+fn paragraph<'a, S, E>(
+	state: &mut State<'a>,
+	pex: &E,
+) -> Result<Paragraph<S, E::Output>, E::Error>
 where
 	S: From<&'a str>,
+	E: ParseExtensions<'a>,
 {
 	let mut pieces = Vec::new();
 
@@ -308,10 +370,10 @@ where
 			}
 			// extension
 			[Some(('@', _)), Some(('{', _))] => {
-				ParagraphPiece::Extension(extension(state)?)
+				ParagraphPiece::Extension(extension::<S, E>(state, pex)?)
 			}
 			// regular text
-			_ => ParagraphPiece::Text(text(state)?),
+			_ => ParagraphPiece::Text(text::<S, E>(state)?),
 		};
 
 		let end = state.current_offset;
@@ -322,9 +384,13 @@ where
 	Ok(Paragraph { pieces })
 }
 
-fn blocks<'a, S>(state: &mut State<'a>) -> Result<Vec<Spanned<Block<S>>>>
+fn blocks<'a, S, E>(
+	state: &mut State<'a>,
+	pex: &E,
+) -> Result<Vec<Spanned<Block<S, E::Output>>>, E::Error>
 where
 	S: From<&'a str>,
+	E: ParseExtensions<'a>,
 {
 	let mut out = Vec::new();
 	while let Some((ch, _)) = state.peek() {
@@ -332,12 +398,8 @@ where
 
 		let start = state.current_offset;
 		let block = match ch {
-			'#' => Block::Header(header(state)?),
-			ch if ch.is_whitespace() => {
-				state.forward();
-				break;
-			}
-			_ => Block::Paragraph(paragraph(state)?),
+			'#' => Block::Header(header::<S, E>(state)?),
+			_ => Block::Paragraph(paragraph(state, pex)?),
 		};
 		let end = state.current_offset;
 
@@ -347,18 +409,56 @@ where
 	Ok(out)
 }
 
-pub fn parse<'a, S>(corpus: &'a str) -> Result<Vec<Spanned<Block<S>>>>
+pub struct ParseExtensionsRaw<S>(core::marker::PhantomData<S>);
+impl<S> ParseExtensionsRaw<S> {
+	fn new() -> Self {
+		Self(Default::default())
+	}
+}
+
+impl<'a, S> ParseExtensions<'a> for ParseExtensionsRaw<S>
 where
 	S: From<&'a str>,
+{
+	type Output = Extension<S>;
+
+	type Error = kor::Never;
+
+	fn parse(
+		&self,
+		content: &'a str,
+		span: Span,
+	) -> core::result::Result<Self::Output, Self::Error> {
+		Ok(Extension(content.into()))
+	}
+}
+
+pub fn parse<'a, S>(
+	corpus: &'a str,
+) -> Result<Vec<Spanned<Block<S, Extension<S>>>>, kor::Never>
+where
+	S: From<&'a str>,
+{
+	parse_extended(corpus, ParseExtensionsRaw::new())
+}
+
+pub fn parse_extended<'a, S, E>(
+	corpus: &'a str,
+	pex: E,
+) -> Result<Vec<Spanned<Block<S, E::Output>>>, E::Error>
+where
+	S: From<&'a str>,
+	E: ParseExtensions<'a>,
 {
 	let mut state = State {
 		corpus,
 		current_offset: 0.into(),
 	};
 
-	blocks(&mut state)
+	blocks(&mut state, &pex)
 }
 
+
 #[cfg(test)]
 mod test {
 	use super::*;
@@ -487,4 +587,12 @@ mod test {
 			)]
 		);
 	}
+
+	#[test]
+	fn complex_test_1() {
+		let corpus = include_str!("../test/hello_mup.mup");
+		let output = parse::<&str>(corpus).unwrap();
+
+		assert_eq!(output, &[]);
+	}
 }
diff --git a/test/hello_mup.mup b/test/hello_mup.mup
new file mode 100644
index 0000000..948f93c
--- /dev/null
+++ b/test/hello_mup.mup
@@ -0,0 +1,8 @@
+# Hello mup!
+
+This is mup, a lightweight markup language with a focus on extensibility.
+Mup can be used for many different types of documents!
+
+This is an example of an @{extension block}@. Extension blocks are
+demarcated in the syntax tree, allowing you to run whatever processing you
+would like on the content!