# File lib/dister/cli.rb, line 94
    def format(operation,format = nil)
      access_core
      ensure_valid_option operation, %w(add rm list), "operation"
      if operation == 'list'
        puts "Available formats:"
        puts VALID_FORMATS
      else
        existing_types = @core.options.build_types || []
        chosen_types = case operation
          when "add"
            ensure_valid_option format, VALID_FORMATS, "format"
            @core.options.build_types = (existing_types + [format]).uniq
          when "rm"
            @core.options.build_types = (existing_types - [format])
          else
            existing_types
          end
        puts "Chosen formats:"
        puts chosen_types
      end
    end