| isfd | isam file descriptor |
| index | index number to select |
ISTRUE, or ISFALSE, in which case the error code is returned by isErrno, or found in isfd->iserrno
isIndex is used to select indexes by number. if index is zero then natural order will be selected, otherwise indexes are numbered starting at one for the primary index.
isIndex retains a current record pointer for each index. after selection the current pointer will be at the record selected when the index was last used.