Select "Start string" in the "Windows" menu. A window appears containing the string from which computation will start in production mode:

Generally the start string is predefined symbol 'S', but sometimes you may want to derive other strings of variables/terminal symbols, especially while debugging a grammar. Type or paste start strings on separate lines in the "Start string" window, then select the one you want by highlighting it with the mouse. If you select no start string then the first one in the window is always used. Since 'S' is already there whenever you start the program you may ignore this "Start string" window until you decide to use alternate start strings.

Start string are stored in the "-se" settings file (see §16).

Another, more intuitive, way of using any expression as a start string is to select it in any window (including the grammar itself) and type cmd-p or select "Play selection" in the "Action" window. Normally this command tries to interpret the selection (using the alphabet), but if the selection contains variables then it is taken as a start string and the grammar is prompted to derive a single item from it.