As an example for a specialized view of PIF data the CLS (C list support) library shall be presented, which was designed to be used by applications who need to manipulate PIF geometries in detail like the graphical interactive PIF data editor (PED). Since the PAL provides an array-oriented view of PIF data, this layer is not well suited for interactive graphic manipulation, where often insertion and deletion of single data items is requested. Therefore a linked-list view of PIF data is demanded by those applications, and this is provided by the CLS library.
The CLS library packs PIF lists into objects and implements generic list-manipulation services applicable to all kinds of those lists. This is much like the single linked list and double linked list classes of the GRS library, however, the CLS library was written well before VOOPS was available, and many VOOPS features supporting object-orientedness are implemented here by hand.
Low-level routines of the CLS library are based on the PIL. As it is the case with other high-level libraries, the CLS' internal list representation is converted from and to PIF upon reading and writing, whereas all high-level CLS routines directly work on the internal data structures.