This issue is important as the user of the library of routines is writing a SPMD program and will primarily be using recursive functions to manipulate the data structures. This implies that all processors will have to recurse through all parts of the structure, whether they actually own data nodes in that part or not. Therefore we want to map the nodes to sets of processors in such a way that a test can be inserted into the code that can spot when a processor does not own any node in a sub structure. This test can be used to kill off unnecessary traversal of the data structure, and promote some degree of parallelisation.