1. A method of operating a data processing system comprising a control screen to provide control of a program, said method comprising: providing a first program graphical user interface (GUI) comprising a first GUI description and a first method for operating on data input to or data output from that said first program GUI; providing a second program GUI comprising a second GUI description and a second method for operating on data input to or data output from that said second program GUI; providing a runtime library having said first and second methods; and causing said data processing system to combine said first GUI description and said second GUI description to provide a compound GUI without recompiling said runtime library.