Add an enum for exit class.

src/parser/exiting.rs

@@ -0,0 +1,10 @@
+pub enum ExitClass {
+    /// Headlines and sections.
+    Document,
+
+    /// Elements who take priority over beta elements when matching.
+    Alpha,
+
+    /// Elements who cede priority to alpha elements when matching.
+    Beta,
+}

src/parser/mod.rs

@@ -3,6 +3,7 @@ mod document;
 mod drawer;
 mod element;
 mod error;
+mod exiting;
 mod footnote_definition;
 mod greater_block;
 mod greater_element;