Closes #40

extension/_test/footnote.txt

@@ -12,7 +12,7 @@ That's some text with a footnote.[^1]
 <ol>
 <li id="fn:1" role="doc-endnote">
 <p>And that's the footnote.</p>
-<p>That's the second paragraph.</p>
+<p>That's the second paragraph.<a href="#fnref:1" class="footnote-backref" role="doc-backlink">&#8617;</a></p>
 </li>
 </ol>
 </section>

extension/ast/footnote.go

@@ -34,6 +34,35 @@ func NewFootnoteLink(index int) *FootnoteLink {
 	}
 }
 
+// A FootnoteBackLink struct represents a link to a footnote of Markdown
+// (PHP Markdown Extra) text.
+type FootnoteBackLink struct {
+	gast.BaseInline
+	Index int
+}
+
+// Dump implements Node.Dump.
+func (n *FootnoteBackLink) Dump(source []byte, level int) {
+	m := map[string]string{}
+	m["Index"] = fmt.Sprintf("%v", n.Index)
+	gast.DumpHelper(n, source, level, m, nil)
+}
+
+// KindFootnoteBackLink is a NodeKind of the FootnoteBackLink node.
+var KindFootnoteBackLink = gast.NewNodeKind("FootnoteBackLink")
+
+// Kind implements Node.Kind.
+func (n *FootnoteBackLink) Kind() gast.NodeKind {
+	return KindFootnoteBackLink
+}
+
+// NewFootnoteBackLink returns a new FootnoteBackLink node.
+func NewFootnoteBackLink(index int) *FootnoteBackLink {
+	return &FootnoteBackLink{
+		Index: index,
+	}
+}
+
 // A Footnote struct represents a footnote of Markdown
 // (PHP Markdown Extra) text.
 type Footnote struct {

extension/footnote.go

@@ -180,6 +180,16 @@ func (a *footnoteASTTransformer) Transform(node *gast.Document, reader text.Read
 		return
 	}
 	pc.Set(footnoteListKey, nil)
+
+	for footnote := list.FirstChild(); footnote != nil; footnote = footnote.NextSibling() {
+		var container gast.Node = footnote
+		if fc := container.LastChild(); fc != nil && gast.IsParagraph(fc) {
+			container = fc
+		}
+		index := footnote.(*ast.Footnote).Index
+		container.AppendChild(container, ast.NewFootnoteBackLink(index))
+	}
+
 	node.AppendChild(node, list)
 }
 
@@ -203,6 +213,7 @@ func NewFootnoteHTMLRenderer(opts ...html.Option) renderer.NodeRenderer {
 // RegisterFuncs implements renderer.NodeRenderer.RegisterFuncs.
 func (r *FootnoteHTMLRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
 	reg.Register(ast.KindFootnoteLink, r.renderFootnoteLink)
+	reg.Register(ast.KindFootnoteBackLink, r.renderFootnoteBackLink)
 	reg.Register(ast.KindFootnote, r.renderFootnote)
 	reg.Register(ast.KindFootnoteList, r.renderFootnoteList)
 }
@@ -222,6 +233,19 @@ func (r *FootnoteHTMLRenderer) renderFootnoteLink(w util.BufWriter, source []byt
 	return gast.WalkContinue, nil
 }
 
+func (r *FootnoteHTMLRenderer) renderFootnoteBackLink(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
+	if entering {
+		n := node.(*ast.FootnoteBackLink)
+		is := strconv.Itoa(n.Index)
+		_, _ = w.WriteString(`<a href="#fnref:`)
+		_, _ = w.WriteString(is)
+		_, _ = w.WriteString(`" class="footnote-backref" role="doc-backlink">`)
+		_, _ = w.WriteString("&#8617;")
+		_, _ = w.WriteString(`</a>`)
+	}
+	return gast.WalkContinue, nil
+}
+
 func (r *FootnoteHTMLRenderer) renderFootnote(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
 	n := node.(*ast.Footnote)
 	is := strconv.Itoa(n.Index)