add label for referencing in semantics book

--- a/src/Doc/ProgProve/Types_and_funs.thy	Wed Mar 20 11:16:31 2013 +0100
+++ b/src/Doc/ProgProve/Types_and_funs.thy	Wed Mar 20 11:32:16 2013 +0100
@@ -16,6 +16,7 @@
 Type synonyms are expanded after parsing and are not present in internal representation and output. They are mere conveniences for the reader.
 
 \subsection{Datatypes}
+\label{sec:datatypes}
 
 The general form of a datatype definition looks like this:
 \begin{quote}