Editing P system (section)

===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.