+ sub cb {
+ ($udata, $key) = @;
+ return "$udata-$key-$udata";
+ }
+ $o->set_callback(\&cb, "xyz");
+ assert($o->option("foo") eq "xyz-foo-xyz");
+
+This method allows a callback function to be installed in an option
+set, so that the values of options can be calculated algorithmically
+rather than, as usual, looked up in a table. Along with the callback
+function itself, an additional datum is provided: when an option is
+subsequently looked up, this datum is passed to the callback function
+along with the key; and its return value is returned to the caller as
+the value of the option.
+
+B<Warning.>
+Although it ought to be possible to specify callback function using
+the C<\&name> syntax above, or a literal C<sub { code }> code
+reference, the complexities of the Perl-internal memory management
+system mean that the function must currently be specified as a string
+containing the fully-qualified name, e.g. C<"main::cb">.>
+
+B<Warning.>
+The current implementation of the this method leaks memory, not only
+when the callback is installed, but on every occasion that it is
+consulted to look up an option value.