fetch (RMail::Header)

    # File rmail/header.rb, line 126
        def fetch(name, *rest)
          if rest.length > 1
            raise ArgumentError, "wrong # of arguments(#{rest.length + 1} for 2)"
          end
          result = self[name]
          if result.nil?
            if block_given?
              yield name
            elsif rest.length == 1
              rest[0]
            else
              raise IndexError, 'name not found'
            end
          else
            result
          end
        end