ROLBFN ( nsteps, asteps, ifail )
================================

  Rolls back or forward by N steps between roll-marks

Receives:
  int                      *nsteps         --- number of steps to be rolled
                                           --- forward (> 0) or back (< 0)

Returns:
  int                      *asteps         --- actual number of steps rolled
                                           --- forward (> 0) or back (< 0)
  KI_cod_error             *ifail          --- error code

Specific errors:
  KI_not_at_rollmark        state of modeller changed since last roll-mark
  KI_roll_forward_fail      Roll forward not possible
  KI_bad_value              'nsteps' is zero
  KI_roll_is_off            logging for roll-back is disabled

Description:
  Rolls the model forward or back by 'nsteps'; a positive number indicates
  roll-forward, a negative number specifies roll-back. Error code
  KI_bad_value will be returned if 'nsteps' is zero.

  Rolling forward or back has the effect of restoring tag memory, modelling
  parameters, interface parameters and the bulletin board to the state they
  were in immediately after the call to ROLSMK which set the relevant
  roll-mark.

  The ROLBFN call will affect validity of tags of entities which were created
  or deleted between the two roll-marks representing the states before and
  after the ROLBFN call. The modeller will always trap invalid tags, giving
  error KI_not_a_tag. Note, however, that entities restored by a get-snapshot
  operation may have the same tags as entities which existed prior to the
  GETSNP call.

  It may be that modeller is unable to roll as many steps as have been
  requested - this may be because not enough marks have yet been set, or
  because the logging file is not large enough to record that many steps,
  or because an attempt is being made to roll forward to a mark which has
  been invalidated by a call to ROLSMK since the roll-back. Whatever the
  reason, the modeller will roll back or forward as many steps as it can,
  and will return the number of steps in 'asteps'. The modeller will not
  raise an error, even when no steps can be rolled (in this case 'asteps'
  will be returned as zero).

  If the state of the modeller has been changed since the most recent call
  to ROLSMK, ROLBFN or ROLBLM, ROLBFN will return the error code
  KI_not_at_rollmark. In this the case, you must either call ROLSMK (to
  set a new roll-mark) or ROLBLM (to roll back to the previous one)
  before calling ROLBFN.

  Only 1024 rollmarks are stored.
  It is not possible to roll backwards or forwards further than the 1024 marks
  set.

  Routine OUSTAT can be used to get information about the roll-back system
  such as how many roll-back and roll-forward steps are available and whether
  the modeller is at a roll-mark.