Update prefixes

As you could see from examples for updating records using WBSP, when new values for database table fields are received, the engine will replace old values with new ones. However in some cases there is a need to store new value that is related to old one (e.g. increased by some value, decreased by some value, etc.), or to delete field value completely by setting new value to empty string (""). Some of these tasks can not be done at all (without these prefixes), and those that can be done require a lot of work to accomplish. Therefore WBSP has defined update prefixes, special values that should be added at the beginning of the WBF_ form fields, before the actual value. The prefixes are: