The algorithms described in this paper form an important component of the Lagrangian framework in the Grid unit of
FLASH. Note that PARAMESH provides no native support for Lagrangian data structures or their movement. The FLASH
Lagrangian framework provides this support without relying upon access to any internal PARAMESH data. Because the same
algorithms are applicable to both active and tracer particles displacement, from here on we describe only tracer particles
without any loss of generality.