Copy/Paste the following code to your GroIMP project:
//**********************************************************
/* You will learn with this example:
– how to use constants,
– how to make the application of a rule depend on a condition.
– Change the values for the constants which are defined at the
beginning and compare the resulting behavior.
*/

module Shoot(float len) extends F(len);
module Bud(float strength) extends Sphere(0.2)
{{setShader(RED); setTransform(0,0,0.3);}};

//—————————————————-/

const float APICAL_REDUCTION_FACTOR = 0.9;
const float STRENGTH_LIMIT = 2;

// Start block with the start word
protected void init()
[ Axiom ==> Bud(5); ]

// public rule block (appears as button in the menue)
public void grow()
[
// condition is set (in parentheses (…) )
b:Bud(x), (b[strength] > STRENGTH_LIMIT) ==> Shoot(x)
[ RU(50) Bud(0.7*x) ] [ RU(-50) Bud(0.7*x) ]
Bud(APICAL_REDUCTION_FACTOR * x);
]