Editing P system (section)

===Rules===
Rules represent a possible chemical reaction within a membrane, causing it to evolve to a new state. A rule has a required set of input objects (symbols or catalysts) that must be present in order for it to be applied. If the required objects are present, it consumes them and produces a set of output objects. A rule may also be specified to have a priority over other rules, in which case less dominant rules will only be applied when it is not possible to apply a more dominant rule (i.e. the required inputs are not present).

There are three (in the basic P system model) distinct ways in which a rule may handle its output objects. Usually, the output objects are passed into the current membrane (the same membrane in which the rule and the inputs reside), known as a ''here'' rule. However, there are two modifiers that can be specified upon output objects when rules are defined, ''in'' and ''out''. The ''in'' modifier causes the object to be passed to one of the current membrane's children (travelling inwards relative to the structure of the P system), chosen at [[Randomness|random]] during the computation. The ''out'' modifier causes the object to be passed out of the current membrane and into either its parent membrane or to a sibling membrane, specified during specification of the P system.