Copy/Paste the following code to your GroIMP project:
//**********************************************************
/* You will learn:
Three ways how to access the properties (attributes) of an object.

1. objectDenominator.attribute (e.g., k.length)
(same syntax as in the programming language Java!)
– this allows to change the attribute “length”, but does not trigger
a redrawing of the scene.
2. objectDenominator[attribute] (e.g., k[length])
– this changes the attribute length and triggers the new drawing
of the scene; the 3D view is actualized.
3. For certain attributes, specific setAttribute() and getAttribute()
methods are implemented.

Note: Attributes which are inherited from superclasses (mother class,
grandmother class etc.) can only be accessed using way # 2.
*/

module Bud() extends Sphere(0.1) {{ setShader(RED); }};

module Internode() extends F(1) {{ setShader(GREEN); setScale(1); }};

protected void init()
[ Axiom ==> Internode() Bud(); ]

// rule block 1: rule application – scene is drawn again.
public void run1()
[
i:Internode ::> i[length] = i[length] + 0.1;
]

// rule block 2: rule application – scene is not drawn again.
// New drawing can be enforced manually by “View -> Display ->
// OpenGL”.
public void run2()
[
i:Internode ::> i.length = i.length + 0.1;
]

// method 3: rule application – scene is drawn again.
// Uses an attribute of a superclass of higher order
// (grand-grandmother).
public void run3()
[
i:Internode ::> i[scale] += 0.1;
]

// method 4: rule application – scene is not drawn again.
// Uses a special set() method.
public void run4()
[
i:Internode ::> i.setScale(1);
]

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