## Implement New Fragmentation Model

In case you want to append or replace any of the Pythia fragmentation models with your own, it is possible using some experimental features, but is not trivial (just as for the parton showers). In general, fragmentation models are expected to take a colour subsystem in an event, and convert the partons of this system into colourless bound hadrons. By default, Pythia runs a sequence of three fragmentation models in the following order: a model for Hidden Valley (HV) scenarios, a model for R-hadron scenarios, and the string fragmentation coupled with the mini-string fragmentation. These models can be switched on or off, via various settings; neither the HV nor R-hadron model is run unless the corresponding processes have been switched on. Typically, the string fragmentation is always run, unless explicitly turned off.

### The FragmentationModel interface

The `pythia.setFragmentationPtr( fragmentationPtr)`, `pythia.addFragmentationPtr( fragmentationPtr)`, and `pythia.insertFragmentationPtr(index, fragmentationPtr)` methods allow the fragmentation model sequence to be completely replaced, appended, or modified with an inserted model, respectively. If the default Pythia string fragmentation models is removed from the fragmentation model sequence, this model will not be used for the standard fragmentation. However, an independent version of this model will still be used elsewhere in Pythia, e.g. for low energy processes. At no point will any of the user provided hadronization models replace this internal model. Each `FragmentationModel` in the sequence is initialized by Pythia during the main Pythia initialization call. Consequently, every time the fragmentation pointer sequence is modified, Pythia must be re-initialized.

virtual bool FragmentationModel::init(StringFlav* flavSelPtr, StringPT* pTSelPtr, StringZ* zSelPtr, FragModPtr fragModPtr)
This method sets the flavor selection, pT selection, z selection, and fragmentation modification pointers. None of these pointers needs be used by the fragmentation model, but in many cases they may be useful. The default constructor for `FragmentationModel` will set the internal `flavSelPtr`, `pTSelPtr`, and `zSelPtr` pointers for the base class. The only other method that needs to be implemented is the main fragmentation method.

virtual bool FragmentationModel::fragment(int iSub, ColConfig& colConfig, Event& event, bool isDiff = false, bool systemRecoil = true)
This method fragments the colour subsystem with index `iSub` with a colour configuration given by `colConfig[iSub]`. The special index `iSub == -1` indicates that the method is expected to fragment the entire event, rather than the specified subsystem. Consequently, it is important that models check the validity of `iSub`, i.e. it is not `-1` befor passing it as an index to `colConfig`. The event record should be modified so that the fragmented partons are no longer final particles, and any particles produced from the fragmentation should be appended to the event record passed by the argument `event`. Some fragmentation models require knowledge whether the event was diffractive, which is specified by the `isDiff` flag. Additionally, for the case of mini-string-like fragmentation, it may be necessary to recoil against a single hadron, rather than the entire system. This is specified by the `systemRecoil` flag.