-- Make [[WikiWords]] (single word, all in one screen line) clickable.
      local trimmed_word = rtrim(frag)  -- compute_fragments puts whitespace at the end
      if starts_with(trimmed_word, '[[') and ends_with(trimmed_word, ']]') then
        local filename = trimmed_word:gsub('^..(.*)..$', '%1')
        if source.link_exists(State, filename) then
          local filename_text = App.newText(love.graphics.getFont(), filename)
          button(State, 'link', {x=x+App.width(to_text('[[')), y=y, w=App.width(filename_text), h=State.line_height, color={1,1,1},
            icon = icon.hyperlink_decoration,
            onpress1 = function()
                         source.switch_to_file(filename)
                        end,
          })
        end
      end

function source.link_exists(State, filename)
  if State.link_cache == nil then
    State.link_cache = {}
  end
  if State.link_cache[filename] == nil then
    State.link_cache[filename] = file_exists(filename)
  end
  return State.link_cache[filename]
end

  State.link_cache = {}

end
function starts_with(s, sub)
  return s:find(sub, 1, --[[no escapes]] true) == 1
end
function ends_with(s, sub)
  return s:reverse():find(sub:reverse(), 1, --[[no escapes]] true) == 1

Hyperlink_decoration_color = {r=0.4, g=0.4, b=1}

end
function icon.hyperlink_decoration(button_params)
  local x,y = button_params.x, button_params.y
  App.color(Hyperlink_decoration_color)
  love.graphics.line(x,y+Editor_state.line_height, x+button_params.w,y+Editor_state.line_height)