at present this routine is written very specifically to handle
the compbination of partials w.r.t. j2 of the moon from a
rotation integration and an orbit integration.  hence, the calling
arguments are minimal, and the tests for whether to call hpart2 in
hpartl includes three specific checks.  this was done in order
to minimize the number of conditions under which the subroutine
is called.  in the future, the routine could be generalized by
adding more arguments to the call list and executing the call
under more circumstantces.