LibMarkdown: Implement introspection of the document tree

Userland/Libraries/LibMarkdown/Forward.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Ben Wiederhake <BenWiederhake.GitHub@gmx.de>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Markdown {
+
+class Block;
+class Document;
+class Text;
+
+class BlockQuote;
+class CodeBlock;
+class ContainerBlock;
+class Heading;
+class HoriziontalRule;
+class List;
+class Paragraph;
+class Table;
+
+class Visitor;
+
+}