rule start { 
  set maxgenerations 2000
  R1
  R2
} 

rule R1 { 
  { x 1 rz 0.1 ry 0.1 s 0.99 } R1
  sphere 
} 

rule R2 { 
  { x -1  rz 0.1 ry 0.1 s 0.99 } R2
  sphere 
}