Create Markup::Element base class

Author: vinistock

lib/rdoc/markup.rb

@@ -210,6 +210,7 @@ def convert(input, formatter)
   autoload :BlankLine,             "#{__dir__}/markup/blank_line"
   autoload :BlockQuote,            "#{__dir__}/markup/block_quote"
   autoload :Document,              "#{__dir__}/markup/document"
+  autoload :Element,               "#{__dir__}/markup/element"
   autoload :HardBreak,             "#{__dir__}/markup/hard_break"
   autoload :Heading,               "#{__dir__}/markup/heading"
   autoload :Include,               "#{__dir__}/markup/include"

lib/rdoc/markup/element.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module RDoc
+  class Markup
+    # Base class defining the interface for all markup elements found in documentation
+    # @abstract
+    class Element
+      # @abstract
+      #: (untyped) -> void
+      def accept(visitor)
+        raise NotImplementedError, "#{self.class} must implement the accept method"
+      end
+
+      # @abstract
+      #: (PP) -> void
+      def pretty_print(q)
+        raise NotImplementedError, "#{self.class} must implement the pretty_print method"
+      end
+    end
+  end
+end