+ * Backend for address book as a pipe separated file
+ *
+ * Stores the address book in a local file
+ *
+ * An array with the following elements must be passed to
+ * the class constructor (elements marked ? are optional):
+ *<pre>
+ * filename => path to addressbook file
+ * ? create => if true: file is created if it does not exist.
+ * ? umask => umask set before opening file.
+ * ? name => name of address book.
+ * ? detect_writeable => detect address book access permissions by
+ * checking file permissions.
+ * ? writeable => allow writing into address book. Used only when
+ * detect_writeable is set to false.
+ * ? listing => enable/disable listing
+ *</pre>
+ * NOTE. This class should not be used directly. Use the
+ * "AddressBook" class instead.