-(defun read-expression (prompt &optional initial-contents history)
- "Return a Lisp object read using the minibuffer.
-Prompt with PROMPT. If non-nil, optional second arg INITIAL-CONTENTS
-is a string to insert in the minibuffer before reading.
-Third arg HISTORY, if non-nil, specifies a history list."
+(defun read-expression (prompt &optional initial-contents history default-value)
+ "Return a Lisp object read using the minibuffer, prompting with PROMPT.
+If non-nil, optional second arg INITIAL-CONTENTS is a string to insert
+ in the minibuffer before reading.
+Third arg HISTORY, if non-nil, specifies a history list.
+Fourth arg DEFAULT-VALUE is the default value. If non-nil, it is used
+ for history command, and as the value to return if the user enters the
+ empty string."