Figure 3 is an algorithm that determines the size of the valid feature selection space in a feature model recursively. Applying this algorithm to the feature model in Figure 1b yields a space of 8 valid feature selections, calculated as follows: 2 from F1 × 2 from F2 × (2 from F3 + 2 from F4 – 2). Without considering the inter-feature relationships to prune the invalid selections, the space of feature selection would have been 2number of features = 24 = 16.