Error messages are descriptive. There's no error message. Modularity and Extensibility Logo programs are usually collections of small procedures. Generally, procedures are defined by writing them in a text editor. The special word to is followed by the name of the procedure.
Subsequent lines form the procedure definition.
The word end signals that you're finished. Once a Logo procedure is defined it works like the Logo primitives. In fact, when you look at Logo programs there's no way of knowing which words are primitives and which are user-defined unless you know that particular Logo implementation. In our language sample we used the procedure pick to randomly select an item from a list, for example in the procedure who. In some versions of Logo pick is a primitive while in others you have to write it yourself.