So far we have seen how BP2 can be used to produce items randomly. BP2 makes decisions on candidate rules and positions of derivation following a (pre-set) pseudo-random sequence generated by the computer, as indicated in §5.1.

Another way of controlling computation is to make stepwise decisions on candidate rules. This becomes possible by selecting option "Choose candidate rule" (see Fig.31 infra). Now BP2 will display computation steps along with the rules used, prompting the user to click "Resume" at each step. When there are several candidate rules the user is prompted to mark the selected one.

To select a rule just click on it, then click "Resume" or type cmd-r. If the "Resume-Undo-Stop" window disappears you may click the upper bar (the drag region) of the "Trace" window to bring it to front.

In the example below rule "X --> a" has been selected:

If your decision leads to an unacceptable solution you may undo the computation typing cmd-z or selecting "Undo" in the "Action" menu. Undoing permits to backtrack as far as possible.

At any stage of the computation you may want to stop and save current choices. Click button "Save decisions". The current grammar is saved along with decisions. Decisions include the index of the subgrammar, the index of the selected rule and the position of the current derivation.

When loading a decision file (clicking "Load decisions") BP2 first checks whether or not the grammar currently in memory is the same one as the grammar to which decisions were refering. (BP2 remembers the date and time at which the grammar was compiled.) If the grammar has changed it suggests to delete the current grammar and replace it with the version saved along with the decision file.

Forcing a set of decisions on a grammar which is no longer compatible may produce unpredictable results including crashing the system .

Once a decision file is loaded, select "Step-by-step compute" and click "Repeat computation". When reaching the point at which the production had been interrupted, the user is prompted to continue making his/her own decisions.