ROLSMK ( ifail )
================

  Sets a roll-back mark

Returns:
  KI_cod_error             *ifail          --- error code

Specific errors:
  KI_rollmark_failed        failed to set rollmark
  KI_roll_is_off            logging for roll-back is disabled

Description:
  Records the state of the model so that a later roll-back (using
  ROLBLM or ROLBFN) may return to it.

  This mark will last until one of the following happens :

      Several later marks are set. The exact number will depend on the
      size of the roll-back file, as set in the call to SEINTP. Eventually
      this mark will not be recoverable, and a call to ROLBFN will return
      a smaller number of steps than requested.

      ROLBFN is called with a sufficient number of steps to roll back
      further than this mark, then another mark is set by again calling
      ROLSMK; a call to ROLSMK invalidates any rollmarks which might
      otherwise be used for a roll-forward.