-- fix ligatures
-- replace ligatures produced by tex4ht with their components
-- this prevents problems with some readers
local gsub = unicode.utf8.gsub
function filter(s)
	local replaces = {
		["fi"] = "fi",
		["ffi"] = "ffi",
		["fl"] = "fl",
		["ffl"] = "ffl",
		["ff"] = "ff"
	}
	return gsub(s, "([fiffiflfflff])",function (x) return replaces[x] or x end)
end

return filter