Copy/Paste the following code to your GroIMP project:
//**********************************************************
/* You will learn:
– to work with imperative XL commands in order to actualize
attributes of an object in actualization rules.
– watch the attributes of the objects in the attribute editor
of GroIMP and the possibilities given there to change these
attributes.
– also possible to have direct access there to attributes of
the superclass of the considered object!
*/

// Definition of a CONSTANT: Constants are often denoted with
// CAPITALS.

const float MAXANGLE = 90.0;

// Definition eines eigenen Rotationsbefehls als Erweiterung von RU:
// Geerbt wird die Eigenschaft “angle” der Oberklasse RU, die mit
// super.angle als Parameter fuer den Konstruktor uebernommen wird.

module Rotation(super.angle) extends RU(angle);

protected void init()
[

// Creation of branching under use of the user-defined
// rotatoin module
Axiom ==> L(10) F0
[ MRel(0.7) Rotation(-30) F(6) ]
[ MRel(0.5) Rotation(40) F(6) ];
]

public void changeAngle()
[

// use of an actualization rule ( ::> )
r:Rotation, (r[angle] < MAXANGLE && r[angle] > -MAXANGLE) ::>

{ // imperative code block
if (r[angle] < 0)
r[angle] -= 3; /* short notation for: r[angle] = r[angle] – 3; */
else
r[angle] += 3; /* analogously */
}
]

// An actualization rule is used to change attributes (properties)
// of objects (i.e., of nodes) by rule application.
// With this rule, no objects are replaced. The structure of the graph
// is not changed.
// Only the attributes of existing objects (nodes of the graph) can be
// actualized.
// Rules of this type are marked by the ::> arrow.

//**********************************************************