Add `switch` command

Change-Id: I5db53925f1710e29b49398621e6b634889f1df15
diff --git a/src/ast_handlers.hpp b/src/ast_handlers.hpp
index 686af85..288801d 100644
--- a/src/ast_handlers.hpp
+++ b/src/ast_handlers.hpp
@@ -270,6 +270,8 @@
 
 struct exec_class;
 
+struct switch_class;
+
 struct cancel_class;
 
 struct command_class {