Document that the |mode| parameter to open() is optional and explain how
the default value is determined.
diff --git a/Doc/lib/libaifc.tex b/Doc/lib/libaifc.tex
index e2e3607..9ba46ed 100644
--- a/Doc/lib/libaifc.tex
+++ b/Doc/lib/libaifc.tex
@@ -33,15 +33,16 @@
Module \module{aifc} defines the following function:
-\begin{funcdesc}{open}{file, mode}
+\begin{funcdesc}{open}{file\optional{, mode}}
Open an AIFF or AIFF-C file and return an object instance with
-methods that are described below. The argument file is either a
-string naming a file or a file object. The mode is either the string
-\code{'r'} when the file must be opened for reading, or \code{'w'}
-when the file must be opened for writing. When used for writing, the
-file object should be seekable, unless you know ahead of time how many
-samples you are going to write in total and use
-\method{writeframesraw()} and \method{setnframes()}.
+methods that are described below. The argument \var{file} is either a
+string naming a file or a file object. \var{mode} must be \code{'r'}
+or \code{'rb'} when the file must be opened for reading, or \code{'w'}
+or \code{'wb'} when the file must be opened for writing. If omitted,
+\code{\var{file}.mode} is used if it exists, otherwise \code{'rb'} is
+used. When used for writing, the file object should be seekable,
+unless you know ahead of time how many samples you are going to write
+in total and use \method{writeframesraw()} and \method{setnframes()}.
\end{funcdesc}
Objects returned by \function{open()} when a file is opened for