If the generative music has created the vision of automated music systems, the developments that really has brought a sufficient independence degree and acoustically appreciable results has come from different areas. Pet_00by Laura Baxter and Simon Yuill is a Flash music sequencer project. The user inserts three types of elements: particles, attractors and triggers. Particles are related to the sounds composed by Laura Baxter and they move across the screen until they face a trigger area, and then they are played. The particles' movements are established by the attractors' position. The sequencing, indeed, is made establishing the elements' rules, with their specific characteristics. So what is built in the editing is a process, that does not involves only the juxtaposition of elements, but also the intuitive writing of the interaction rules. This dynamic sonic ecosystems made out of checkable behaviors and it builds self-playing scores, that visually show their components' actions.