The Hipe API
To write a graphical program for Hipe, you'll need to include the Hipe API in your program. The Hipe API is designed to be used with C and C++ programs. The Hipe API is designed to be compact and general rather than user-friendly. The idea is that if user-friendliness or a particular GUI-development approach is required in future, it will be easy to build the necessary GUI toolkit on top of the Hipe API in future.
You can modify and include the Hipe API freely in your code even if you write proprietary software, subject to the license conditions.