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diff --git a/src/construct/gfm_label_start_footnote.rs b/src/construct/gfm_label_start_footnote.rs
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+//! Label start (footnote) occurs in the [text][] content type.
+//!
+//! ## Grammar
+//!
+//! Label start (footnote) forms with the following BNF
+//! (<small>see [construct][crate::construct] for character groups</small>):
+//!
+//! ```bnf
+//! gfm_label_start_footnote ::= '[' '^'
+//! ```
+//!
+//! ## HTML
+//!
+//! Label start (footnote) does not, on its own, relate to anything in HTML.
+//! When matched with a [label end][label_end], they together relate to `<sup>`
+//! and `<a>` elements in HTML.
+//! See [*§ 4.5.19 The `sub` and `sup` elements*][html_sup] and
+//! [*§ 4.5.1 The `a` element*][html_a] in the HTML spec for more info.
+//! Without an end, the characters (`[^`) are output.
+//!
+//! ## Tokens
+//!
+//! *   [`LabelImage`][Name::LabelImage]
+//! *   To do.
+//!
+//! ## References
+//!
+//! *   [`micromark-extension-gfm-footnote`](https://github.com/micromark/micromark-extension-gfm-footnote)
+//!
+//! > 👉 **Note**: Footnotes are not specified in GFM yet.
+//! > See [`github/cmark-gfm#270`](https://github.com/github/cmark-gfm/issues/270)
+//! > for the related issue.
+//!
+//! [text]: crate::construct::text
+//! [label_end]: crate::construct::label_end
+//! [html_a]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element
+//! [html_sup]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-sub-and-sup-elements
+
+use crate::event::Name;
+use crate::resolve::Name as ResolveName;
+use crate::state::{Name as StateName, State};
+use crate::tokenizer::{LabelKind, LabelStart, Tokenizer};
+
+/// Start of label (footnote) start.
+///
+/// ```markdown
+/// > | a [^b] c
+///       ^
+/// ```
+pub fn start(tokenizer: &mut Tokenizer) -> State {
+    if tokenizer
+        .parse_state
+        .options
+        .constructs
+        .gfm_label_start_footnote
+        && tokenizer.current == Some(b'[')
+    {
+        tokenizer.enter(Name::GfmFootnoteCallLabel);
+        tokenizer.enter(Name::LabelMarker);
+        tokenizer.consume();
+        tokenizer.exit(Name::LabelMarker);
+        State::Next(StateName::GfmLabelStartFootnoteOpen)
+    } else {
+        State::Nok
+    }
+}
+
+/// After `[`, at `^`.
+///
+/// ```markdown
+/// > | a [^b] c
+///        ^
+/// ```
+pub fn open(tokenizer: &mut Tokenizer) -> State {
+    match tokenizer.current {
+        Some(b'^') => {
+            tokenizer.enter(Name::GfmFootnoteCallMarker);
+            tokenizer.consume();
+            tokenizer.exit(Name::GfmFootnoteCallMarker);
+            tokenizer.exit(Name::GfmFootnoteCallLabel);
+            tokenizer.tokenize_state.label_starts.push(LabelStart {
+                kind: LabelKind::GfmFootnote,
+                start: (tokenizer.events.len() - 6, tokenizer.events.len() - 1),
+                inactive: false,
+            });
+            tokenizer.register_resolver_before(ResolveName::Label);
+            State::Ok
+        }
+        _ => State::Nok,
+    }
+}