Tripartitioning.

  The 'Background' section of the assignment refers to three vectors i, j, and
  k.  Can we assume that the first parameter for tripartition() is always
  vector i, the second is always vector j and the third is always vector k?

The vectors' names don't matter; tripartitioning is a property of the vectors'
contents.  Three vectors, whatever you want to call them, are tripartitioned if
(and only if) one of the vectors contains only values congruent to 0 mod 3,
another of the vectors contains only values congruent to 1 mod 3, and the third
vector contains only values congruent to 2 mod 3.  (This definition has to be
cleaned up a bit to deal with empty vectors, but that would just make things
more complicated without being relevant to the question.)