Added ML reference

--- a/doc-src/Ref/ref.bbl	Thu Jul 11 15:02:42 1996 +0200
+++ b/doc-src/Ref/ref.bbl	Thu Jul 11 15:13:52 1996 +0200
@@ -49,6 +49,11 @@
 \newblock {\em {ML} for the Working Programmer}.
 \newblock Cambridge University Press, 1991.
 
+\bibitem{paulson-ml2}
+Lawrence~C. Paulson.
+\newblock {\em {ML} for the Working Programmer}.
+\newblock Cambridge University Press, 2nd edition, 1996.
+
 \bibitem{pelletier86}
 F.~J. Pelletier.
 \newblock Seventy-five problems for testing automatic theorem provers.