# File lib/asciidoctor/converter/factory.rb, line 45
        def default initialize_singleton = true
          return @__default__ || new unless initialize_singleton
          # FIXME this assignment is not thread_safe, may need to use a ::Threadsafe helper here
          @__default__ ||= begin
            require 'thread_safe'.to_s unless defined? ::ThreadSafe
            new ::ThreadSafe::Cache.new
          rescue ::LoadError
            warn 'asciidoctor: WARNING: gem \'thread_safe\' is not installed. This gem is recommended when registering custom converters.'
            new
          end
        end