In order to judge the effects of the definitions, and to verify that the evolution is following the intended course, several degrees of trial evolution are offered as options.
The choice of the two symbols ? and / was not particularly elegant---they sit at the lower right hand corner of the keyboard, where they are readily accessible.
Point evolution consults the rule table to find out what the cell under the cursor will become, if no changes are contemplated. It is useful for checking that a planned transition is consistent with those already defined.
Unconditional evolution, via ?, is the variant which processes the whole line at once, rather than individual cells. The table is taken as given, and followed unquestioningly. Conditional evolution, on the other hand, inserts an ``undefined'' marker for each unmarked transition in the table. The marker is usually k for a k state automaton, for which the only legitimate symbols are 0, 1, ..., k-1.
By this mechanism, transitions which have yet to be defined stand clearly in evidence. If any ancestor of a cell is undefined, the cell will be undefined also; conditional evolution may be safely followed through several lines.