-- Rewrite in-document links so catalog PDFs point at other PDFs, not .md/.html.
function Link(el)
  local t = el.target or ""
  if t:match("^https?://") and not t:match("zapier%.georgelambert%.org") then
    return el
  end
  if t:match("^mailto:") or t:match("^#") then
    return el
  end
  local path, frag = t:match("^(.-)(#.*)$")
  if not path then
    path, frag = t, ""
  end
  -- PDFs should follow other PDFs for markdown sources. Keep .html (indexes, sphinx).
  if path:match("%.md$") then
    path = path:gsub("%.md$", ".pdf")
    el.target = path .. frag
  end
  return el
end