Abstract: Means are provided which aid in generating programs for testing digital modules. Formal hierarchical descriptions (e.g. in Prolog) of the module without and with a defined fault are stored in a storage device comprising two stores backed up by a submodule store. The two moldule descriptions are fed into registers. Then, repeatedly, (a) an unfolding processor works down the hierarchy, (b) a simplification processor simplifies the descriptions using a variety of logical and arithmetical rules, and (c) a discrimination condition extraction processor extracts discrimination conditions which are common to both module descriptions and necessary (i.e. must be satisfied as part of the final test programme). Another processor has discrimination condition storage means and discrimination condition consistency checking means coupled to it. The processors use common rules, stored in a logical rules store, which rely heavily on expressions of the if-then-else form.