--- a/doc/interpreter/var.txi	Fri Sep 14 05:21:57 2007 +0000
+++ b/doc/interpreter/var.txi	Fri Sep 14 15:17:53 2007 +0000
@@ -171,9 +171,29 @@
 variables is that persistent variables are local in scope to a
 particular function and are not visible elsewhere.
 
-A variable may be declared persistent using a @code{persistent}
-declaration statement.  The following statements are all persistent
-declarations.
+The following example uses a persistent variable to create a function
+that prints the number of times it has been called.
+
+@example
+@group
+function count_calls ()
+  persistent calls = 0;
+  printf ("'count_calls' has been called %d times\n", ++calls);
+endfunction
+
+for i = 1:3
+  count_calls ();
+endfor
+
+@print{} 'count_calls' has been called 1 times
+@print{} 'count_calls' has been called 2 times
+@print{} 'count_calls' has been called 3 times
+@end group
+@end example
+
+As the example shows, a variable may be declared persistent using a
+@code{persistent} declaration statement.  The following statements are
+all persistent declarations.
 
 @example
 @group
@@ -186,8 +206,9 @@
 
 The behavior of persistent variables is equivalent to the behavior of
 static variables in C. The command @code{static} in octave is also
-recognized and is equivalent to @code{persistent}. Like global
-variables, a persistent variable may only be initialized once.
+recognized and is equivalent to @code{persistent}.
+
+Like global variables, a persistent variable may only be initialized once.
 For example, after executing the following code
 
 @example
@@ -200,6 +221,72 @@
 @noindent
 the value of the persistent variable @code{pvar} is 1, not 2.
 
+If a persistent variable is declared but not initialized to a specific
+value, it will contain an empty matrix.  So, it is also possible to
+initialize a persistent variable by checking whether it is empty, as the
+following example illustrates.
+
+@example
+@group
+function count_calls ()
+  persistent calls;
+  if (isempty (calls))
+    calls = 0;
+  endif
+  printf ("'count_calls' has been called %d times\n", ++calls);
+endfunction
+@end group
+@end example
+
+@noindent
+This implementation behaves in exactly the same way as the previous
+implementation of @code{count_calls}.
+
+The value of a persistent variable is kept in memory until it is
+explicitly cleared.  Assuming that the implementation of @code{count_calls}
+is saved on disc, we get the following behaviour.
+
+@example
+@group
+for i = 1:2
+  count_calls ();
+endfor
+@print{} 'count_calls' has been called 1 times
+@print{} 'count_calls' has been called 2 times
+
+clear
+for i = 1:2
+  count_calls();
+endfor
+@print{} 'count_calls' has been called 3 times
+@print{} 'count_calls' has been called 4 times
+
+clear all
+for i = 1:2
+  count_calls();
+endfor
+@print{} 'count_calls' has been called 1 times
+@print{} 'count_calls' has been called 2 times
+
+clear count_calls
+for i = 1:2
+  count_calls();
+endfor
+@print{} 'count_calls' has been called 1 times
+@print{} 'count_calls' has been called 2 times
+@end group
+@end example
+
+@noindent
+That is, the persistent variable is only removed from memory when the
+function containing the variable is removed.  Note that if the function
+definition is typed directly into the Octave prompt, the persistent
+variable will be cleared by a simple @code{clear} command as the entire
+function definition will be removed from memory.  If you do not want
+a persistent variable to be removed from memory even if the function is
+cleared, you should use the @code{mlock} function as described in
+@xref{Function Locking}.
+
 @node Status of Variables
 @section Status of Variables