A computer-aided method (FIG. 12) for developing software by which programmers may implement different features of an application as independent reusable components, even though these features affect the control flows of one another. A programmer may add new features to an application without the labor intensive tasks of examining and modifying existing code. The programmer uses a set of programming language constructs to specify nonprocedural program units, organize the program units into reusable features and integrate several features together into a feature package. An interaction detection algorithm analyzes program units and determines whether there is interaction among program units in a feature and among features in a feature package. The programmer must resolve the interaction, if detected, before the programs can be executed. A run time system specification supports the semantics of the programming language constructs and preserves the conditions that enable interaction detection.