Relational growth grammars describe an abstract concept. The programming language XL is a concrete implementation of this concept. All examples of the tutorial are written in XL, so you already have seen some XL source code. The purpose of this tutorial is not to give a full specification of the XL programming language, for this the reader is referred to the XL language specification at https://manual.grogra.de/XL/.

Perhaps the most important feature of XL is that it is an extension of Java: Everything that can be written in Java can be written in XL, too. Every existing library of Java classes can be imported and used. You should read at least some introductory literature on Java in order to get familiar with this part of XL. In the sequel, the tutorial will make use of terms of the Java language like classes, instances, fields, methods, and constructors.

Let us reconsider the very first example of this tutorial. Its XL source code looked like this:

public void derivation() [
    Axiom ==> F RU(120) F RU(120) F;
    F ==> F RU(-60) F RU(120) F RU(-60) F;
]

Having said that XL is an extension of Java, the parts of this code can now be seen from the perspective of Java: In Java, methods can be declared as in

public void derivation() {
    doSomething();
}

In XL, this is the same, so the example declares a method named derivation. However, in XL there is the possibility to write a block enclosed in square brackets [] whereever Java allows just a block enclosed in curly braces {}. Within a block enclosed in square brackets, rules are specified, whereas a block enclosed in curly braces contains conventional Java statements.

As has been said earlier in this tutorial, the names Axiom, F, RU correspond to Java classes of that name (in this case in the package de.grogra.turtle). The left hand side of the rule Axiom ==> F RU(120) F RU(120) F; has to be understood as a pattern which matches for every instance of the class Axiom. Now for every such instance which can be found in the graph, the replacement operator ==> replaces the instance by the right hand side, which in this case consists of a linear graph of five nodes. These nodes are newly created ones, because F and RU(120) are interpreted as constructor invocations (in Java, one would write new F() and new RU(120)). The new nodes are connected by a default successor edge which indicates that they are simply written one after another.