There are circumstances in which it is convenient to be able to specify alternative vectors of expectations (and alternative covariance matrices) for elements. Thus, for both expectations and variance-covariance specifications, we allow a number of alternative stores - although the two kinds of store mostly work completely independently. For simplicity and backwards compatibility we also introduce the notion of default expectation store. The number of expectation stores is generally fixed: it is the same as the number of belief stores, and this can be discovered by executing the LOOK: command with argument program ; for some current versions of [B/D] it is 6. The default expectation store number is number one. Most of the commands which reference element expectations work with expectations from the default store number. Such commands include the ADJUST: , SCAN: , ITADJUST: , and COMPARE: commands. Other commands, essentially those involved in defining or modifying expectations or covariances - such as the BUILD: , COBUILD: ,CENTRE: and PRODUCT: commands - deal specifically with individual expectation stores.
A brief summary of the commands and controls affecting expectation stores is as follows. Alternative expectation stores may be locked or unlocked in manner similar to belief stores by using the ELOCK: command. The default expectation store number can be changed using the e argument to the CONTROL: command. Expectations from the various stores can be examined using the e argument to the LOOK: command, whilst this and the the nc argument reveal which expectation store is currently the default. Expectations from alternative stores can be accessed via the ex operator. Adjusted expectations resulting from ADJUST: or SCAN: commands can optionally be retained in an expectation store using the ae and scae controls. Components may have different functional expectations specified in different expectation stores using the FE: command. The EXP: command is used to define expectations for alternative expectation stores. Expectations from any given expectation store can be exported using the EXPORT: command.