| isfd | isam file descriptor |
| recnum | record number to be replaced |
| data | image to replace the given record |
ISTRUE, or ISFALSE, in which case the error code is returned by isErrno, or found in isfd->iserrno
the contents of the given record in the data file will be replaced with the image passed, and all related indexes updated.
the original data image can be found in isfd->data on return