When we talk about free software, one of the issues that most often get discussed is whether it is possible to integrate free software with non-free ones. The original GNU GPL, with the goal in mind that free software should always be free, does not allow the integration of free software with non-free ones, except in the sole case when such integration is only a simple aggregation. This is designed to prevent free software from becoming non-free software after such integration and hence making it no longer free. Many licenses after the GPL do not necessarily impose such restrictions. Some of them even have relatively relaxed requirements in the hope that free software can take a bigger market share and the idea of free software can be more widespread. Different licenses have usually different, detailed terms on this. Issue of compatibility, integration and combination is not limited to the interaction between two free software programs. Such issue also exists between free and non-free software programs. The derivative work that results from the two programs brings forth issues such as the choice of license terms and compatibility. For example, if programs A and B are licensed under licenses A’ and B’ respectively, and programs A and B are combined into the program C. Because program C is the derivative work of both programs A and B, C is bound to the terms of A’ and B’. If A’ and B’ are not compatible, the effectiveness of the integration of A and B will be affected. For detailed discussion, please refer to the section “Compatibility”.