Add zlib
diff --git a/Doc/lib/libsomeos.tex b/Doc/lib/libsomeos.tex
index 49c1ab2..c04cf7a 100644
--- a/Doc/lib/libsomeos.tex
+++ b/Doc/lib/libsomeos.tex
@@ -22,7 +22,14 @@
 
 \item[anydbm]
 --- Generic interface to DBM-style database modules.
+
 \item[whichdbm]
 --- Guess which DBM-style module created a given database.
 
+\item[zlib]
+\item[gzip]
+--- Compression and decompression compatible with the
+\code{gzip} program (zlib is the low-level interface, gzip the
+high-level one).
+
 \end{description}