    # File lib/compass/commands/watch_project.rb, line 129
          def remove_obsolete_css(base = nil, relative = nil)
            compiler = new_compiler_instance(:quiet => true)
            sass_files = compiler.sass_files
            deleted_sass_files = (last_sass_files || []) - sass_files
            deleted_sass_files.each do |deleted_sass_file|
              css_file = compiler.corresponding_css_file(deleted_sass_file)
              remove(css_file) if File.exists?(css_file)
            end
            self.last_sass_files = sass_files
          end