head_sub : head_sour_sect { OCCUR2(SOUR, 1, 1) }
| head_dest_sect { OCCUR2(DEST, 0, 1) }
| head_date_sect { OCCUR2(DATE, 0, 1) }
head_sub : head_sour_sect { OCCUR2(SOUR, 1, 1) }
| head_dest_sect { OCCUR2(DEST, 0, 1) }
| head_date_sect { OCCUR2(DATE, 0, 1) }