+=head2 C<is_type TYPE>
+
+Returns true if the order is of the given type.
+
+=head2 C<convert_to_invoice %params>
+
+Creates a new invoice with C<$self> as the basis by calling
+L<SL::DB::Invoice::new_from>. That invoice is posted, and C<$self> is
+linked to the new invoice via L<SL::DB::RecordLink>. C<$self>'s
+C<closed> attribute is set to C<true>, and C<$self> is saved.
+
+The arguments in C<%params> are passed to L<SL::DB::Invoice::post>.
+
+Returns the new invoice instance on success and C<undef> on
+failure. The whole process is run inside a transaction. On failure
+nothing is created or changed in the database.
+
+At the moment only sales quotations and sales orders can be converted.
+
+=head2 C<create_sales_process>
+
+Creates and saves a new sales process. Can only be called for sales
+orders.
+
+The newly created process will be linked bidirectionally to both
+C<$self> and to all sales quotations that are linked to C<$self>.