Allow ask-parameters to be used as operands

[rensink]

Currently, ask-parameters can only be used as-is; within the rule, no operation is allowed on them. This is more restricting than it should be from a user point of view. Implementation-wise, the main issue standing in the way of using ask-values in calculations is that divide-by-zero is undefined, hence if a user were to input zero and that is used as a divisor, the rule would not be applicable. Currently there is no way to declare a rule to be non-applicable once a match has been found.

The set of partially defined operations may increase at some point, e.g., user-defined operations cannot be statically guaranteed to be complete.

Probably the best solution would be to change the treatment of partial operations so that they generate an error value rather than stopping the rule from being applicable (essentially making them complete operations on an extended domain), and marking any state with an error node as erroneous.

Reported by: rensink

[rensink]

The suggestion in the last paragraph has been implemented: operations are partially defined, an undefined application leads to an error state - see this rule system

dividebyzero.gps.zip