added Gentzen:1935;

--- a/doc-src/manual.bib	Tue Sep 12 07:49:07 2006 +0200
+++ b/doc-src/manual.bib	Tue Sep 12 12:12:25 2006 +0200
@@ -380,6 +380,13 @@
   publisher	= {Wiley},
   year		= 1990}
 
+@Article{Gentzen:1935,
+  author =       {G. Gentzen},
+  title =        {Untersuchungen {\"u}ber das logische {S}chlie{\ss}en},
+  journal =      {Math. Zeitschrift},
+  year =         1935
+}
+
 @InProceedings{gimenez-codifying,
   author	= {Eduardo Gim{\'e}nez},
   title		= {Codifying Guarded Definitions with Recursive Schemes},