def rel_to(path)
      path = path + '/' unless path.end_with?('/')
      if rel.start_with?(path)
        rel.sub(path, '')
      else
        raise "#{@abs} is not in #{path}"
      end
    end
def rel_to(path)
      path = path + '/' unless path.end_with?('/')
      if rel.start_with?(path)
        rel.sub(path, '')
      else
        raise "#{@abs} is not in #{path}"
      end
    end
Pass in a path relative to Ichiban.projet_root. Returns the path relative to the
 one you passed in. E.g. if the file is assets/js/example.js, then
 rel_to('assets/js') returns 'example.js'.