Methods and apparatus, including computer program products, for accepting user input defining a first additional parameter to a first source code component, the first source code component not being modified. Accepting user input defining a first modification to the first source code component, the first modification dependent on the first additional parameter, and the source code component not being modified. Incorporating into a second source code component the first source code component and an expression of the first additional parameter and the first modification, the second source code component being separate from the first source code component. And creating a compiled program using the second source code component.