EPUB: Fix broken links

This is done with a lua filter that rewrites links with regular
expressions.

References #480.

Makefile

@@ -61,6 +61,7 @@ out/plfa.epub: out/ $(AGDA) $(LUA) epub/main.css
 		--epub-embed-font='assets/fonts/FreeMono.woff' \
 		--epub-embed-font='assets/fonts/DejaVuSansMono.woff' \
 		--lua-filter epub/include-files.lua \
+		--lua-filter epub/rewrite-links.lua \
 		--lua-filter epub/default-code-class.lua -M default-code-class=agda \
 		--standalone \
 		--fail-if-warnings \

epub/rewrite-links.lua

@@ -0,0 +1,43 @@
+local currentChapter = nil
+
+-- Reassigns identifiers for all all Headers level 2 and higher. Level 2 Headers
+-- correspond to chapters, and are identified by the first word in their content
+-- field. Headers of level more than 2 are identified by "#<chapter>-<anchor>",
+-- where "<chapter>" is the identifier of the chapter this header is nested in,
+-- and "<anchor>" is this Header's existing identifier.
+function Header (el)
+    if el.level == 2 then
+        local title = pandoc.utils.stringify(el.content[1])
+        currentChapter = title:match("%w+")
+        el.identifier = currentChapter
+    elseif el.level > 2 then
+        el.identifier = currentChapter .. '-' .. el.identifier
+    end
+    return el
+end
+
+
+
+-- Performs the following transformations on Link targets:
+--
+--     Case 1:
+--         [text]({{ site.baseurl }}/chapter/#more-stuff) -> [text](#chapter-more-stuff)
+--
+--     Case 2:
+--         [text]({{ site.baseurl }}/chapter/)            -> [text](#stuff)
+--
+-- All other Links are ignored.
+function Link (el)
+    local n
+    -- When parsing a markdown link such as "[stuff]({{ site.baseurl }}/Naturals",
+    -- pandoc encodes the link's URL with percent-encoding, resulting in the
+    -- link "[stuff](%7B%7B+site.baseurl+%7D%7D%2FNaturals)". This is why the
+    -- pattern begins with ".*site%.baseurl.*" and not "{{ site%.baseurl }}".
+    el.target, n = el.target:gsub("^.*site%.baseurl.*/(%w+)/#([%w-]+)$",  -- case 1
+                                  "#%1-%2")
+    if n == 0 then
+        el.target = el.target:gsub("^.*site%.baseurl.*/(%w+)/$",          -- case 2
+                                "#%1")
+    end
+    return el
+end

src/plfa/part2/More.lagda.md

@@ -1234,7 +1234,7 @@ side to be well typed.
 
 ## Test examples
 
-We repeat the [test examples]({{ site.baseurl }}/DeBruijn/#examples) from Chapter [DeBruijn]({{ site.baseurl }}/DeBruijn),
+We repeat the [test examples]({{ site.baseurl }}/DeBruijn/#examples) from Chapter [DeBruijn]({{ site.baseurl }}/DeBruijn/),
 in order to make sure we have not broken anything in the process of extending our base calculus.
 ```
 two : ∀ {Γ} → Γ ⊢ `ℕ