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) with a display on said control screen, said first program 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 with a display on said control screen, said 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 with a display on said control screen without recompiling said runtime library, wherein said first GUI is displayed by a first program and said second GUI is displayed by a second program, said first GUI description and said second GUI description being combined while programs operating said GUIs are operating.