Figure 4.

Example of the language hierarchy. The four images show the reaction graphs produced for the program (<tû x y> | {tû *}[x]:[y uû]) for the four possible values of level(σ), where unproductive(σ)=leaks(σ)=false. Edges with just a hollow arrowhead indicate irreversible reactions, whereas those which also have a solid arrowhead on the other end denote reversible reactions. The complexity of the reaction graphs decreases from left to right as we move from more detailed to more abstract semantic models of the DNA interactions.