Editing P system (section)

==Components of a P system==
Although many varieties of P system exist, most share the same basic components. Each element has a specific role to play, and each has a founding in the biological cell architecture upon which P systems are based.

===The environment===
The environment is the surroundings of the P system. In the initial state of a P system it contains only the container-membrane, and while the environment can never hold rules, it may have objects passed into it during the computation. The objects found within the environment at the end of the computation constitute all or part of its “result.”

===Membranes===
Membranes are the main “structures” within a P system. A membrane is a discrete unit which can contain a set of objects (symbols/catalysts), a set of rules, and a set of other membranes contained within. The outermost membrane, held within the environment, is often referred to as the 'container membrane' or 'skin membrane'. As implied to by their namesake, membranes are [[Semipermeable membrane|permeable]] and symbols resulting from a rule may cross them. A membrane (but not the container membrane) may also “dissolve”, in which case its content, except for rules (which are lost), migrate into the membrane in which it was contained.<ref name=paun2002 />

Some P system variants allow for a membrane to divide, possess a [[Electric charge|charge]] or have varying [[Semipermeable membrane|permeability]] by changing membrane thickness.<ref name=paun2002 />

===Symbols===
Symbols represent chemicals that may react with other chemicals to form some product. In a P system, each type of symbol is typically represented by a different letter. The symbol content of a membrane is therefore represented by a string of letters. Because the [[Multiplicity (mathematics)|multiplicity]] of symbols in a region matters, [[multisets]] are commonly used to represent the symbol content of a region.

Special case symbols exist, for example, a lower case [[Delta (letter)|delta]] (δ) is often used to initiate the dissolving of a membrane, and this will only ever be found in the output of a rule: upon being encountered it invokes a reaction, and is used in the process.

====Catalysts====
Catalysts are similar to their namesakes in chemistry. They are represented and used in the same way as symbols, but are never consumed during a “[[chemical reaction|reaction]],” they are simply a requirement for it to occur.

===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.