question: public void stepSystem ( int nstep ) { root = null ; makeTree ( nstep ) ; for ( Enumeration < Body > e = bodyTabRev . elementsRev ( ) ; e . hasMoreElements ( ) ; ) { Body b = e . nextElement ( ) ; b . hackGravity ( rsize , root ) ; } vp ( bodyTabRev , nstep ) ; }
target:  Advance the N-body system one time-step.