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diff --git a/src/construct/heading_atx.rs b/src/construct/heading_atx.rs
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+//! Heading (atx) is a construct that occurs in the flow content type.
+//!
+//! They’re formed with the following BNF:
+//!
+//! ```bnf
+//! heading_atx ::= 1*6'#' [ 1*space_or_tab code [ 1*space_or_tab 1*'#' ] ] *space_or_tab
+//!
+//! code ::= . ; any unicode code point (other than line endings).
+//! space_or_tab ::= ' ' | '\t'
+//! ```
+//!
+//! Headings in markdown relate to the `<h1>` through `<h6>` elements in HTML.
+//! See [*§ 4.3.6 The `h1`, `h2`, `h3`, `h4`, `h5`, and `h6` elements* in the
+//! HTML spec][html] for more info.
+//!
+//! `CommonMark` introduced the requirement on whitespace existing after the
+//! opening sequence and before text.
+//! In older markdown versions, this was not required, and headings would form
+//! without it.
+//!
+//! In markdown, it is also possible to create headings with the setext heading
+//! construct.
+//! The benefit of setext headings is that their text can include line endings.
+//! However, their limit is that they cannot form `<h3>` through `<h6>`
+//! headings.
+//! Due to this limitation, it is recommended to use atx headings.
+//!
+//! > 🏛 **Background**: the word *setext* originates from a small markup
+//! > language by Ian Feldman from 1991.
+//! > See [*§ Setext* on Wikipedia][wiki-setext] for more info.
+//! > The word *atx* originates from a tiny markup language by Aaron Swartz
+//! > from 2002.
+//! > See [*§ atx, the true structured text format* on `aaronsw.com`][atx] for
+//! > more info.
+//!
+//! ## References
+//!
+//! *   [`heading-atx.js` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-core-commonmark/dev/lib/heading-atx.js)
+//! *   [*§ 4.2 ATX headings* in `CommonMark`](https://spec.commonmark.org/0.30/#atx-headings)
+//!
+//! [html]: https://html.spec.whatwg.org/multipage/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
+//! [wiki-setext]: https://en.wikipedia.org/wiki/Setext
+//! [atx]: http://www.aaronsw.com/2002/atx/
+//!
+//! <!-- To do: link `flow`, `setext` -->
+
+use crate::constant::HEADING_ATX_OPENING_FENCE_SIZE_MAX;
+use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
+
+/// Start of a heading (atx).
+///
+/// ```markdown
+/// |## alpha
+/// ```
+pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    if Code::Char('#') == code {
+        tokenizer.enter(TokenType::AtxHeading);
+        tokenizer.enter(TokenType::AtxHeadingSequence);
+        sequence_open(tokenizer, code, 0)
+    } else {
+        (State::Nok, None)
+    }
+}
+
+/// In the opening sequence.
+///
+/// ```markdown
+/// #|# alpha
+/// ```
+fn sequence_open(tokenizer: &mut Tokenizer, code: Code, rank: usize) -> StateFnResult {
+    match code {
+        Code::None
+        | Code::CarriageReturnLineFeed
+        | Code::VirtualSpace
+        | Code::Char('\t' | '\n' | '\r' | ' ')
+            if rank > 0 =>
+        {
+            tokenizer.exit(TokenType::AtxHeadingSequence);
+            at_break(tokenizer, code)
+        }
+        Code::Char('#') if rank < HEADING_ATX_OPENING_FENCE_SIZE_MAX => {
+            tokenizer.consume(code);
+            (
+                State::Fn(Box::new(move |tokenizer, code| {
+                    sequence_open(tokenizer, code, rank + 1)
+                })),
+                None,
+            )
+        }
+        _ => (State::Nok, None),
+    }
+}
+
+/// After something but before something else.
+///
+/// ```markdown
+/// ## |alpha
+/// ## alpha| bravo
+/// ## alpha |bravo
+/// ## alpha bravo|##
+/// ## alpha bravo ##|
+/// ```
+fn at_break(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            tokenizer.exit(TokenType::AtxHeading);
+            (State::Ok, Some(vec![code]))
+        }
+        Code::VirtualSpace | Code::Char('\t' | ' ') => {
+            tokenizer.enter(TokenType::AtxHeadingWhitespace);
+            whitespace(tokenizer, code)
+        }
+        Code::Char('#') => {
+            tokenizer.enter(TokenType::AtxHeadingSequence);
+            further_sequence(tokenizer, code)
+        }
+        Code::Char(_) => {
+            tokenizer.enter(TokenType::AtxHeadingText);
+            data(tokenizer, code)
+        }
+    }
+}
+
+/// In a further sequence (after whitespace).
+/// Could be normal “visible” hashes in the heading or a final sequence.
+///
+/// ```markdown
+/// ## alpha #|#
+/// ```
+fn further_sequence(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    if let Code::Char('#') = code {
+        tokenizer.consume(code);
+        (State::Fn(Box::new(further_sequence)), None)
+    } else {
+        tokenizer.exit(TokenType::AtxHeadingSequence);
+        at_break(tokenizer, code)
+    }
+}
+
+/// In whitespace.
+///
+/// ```markdown
+/// ## alpha | bravo
+/// ```
+fn whitespace(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::VirtualSpace | Code::Char('\t' | ' ') => {
+            tokenizer.consume(code);
+            (State::Fn(Box::new(whitespace)), None)
+        }
+        _ => {
+            tokenizer.exit(TokenType::AtxHeadingWhitespace);
+            at_break(tokenizer, code)
+        }
+    }
+}
+
+/// In text.
+///
+/// ```markdown
+/// ## al|pha
+/// ```
+fn data(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        // Note: `#` for closing sequence must be preceded by whitespace, otherwise it’s just text.
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\t' | '\n' | '\r' | ' ') => {
+            tokenizer.exit(TokenType::AtxHeadingText);
+            at_break(tokenizer, code)
+        }
+        _ => {
+            tokenizer.consume(code);
+            (State::Fn(Box::new(data)), None)
+        }
+    }
+}