Define three different ways to proceed insertion. This configures how overlapping data is treated. - NCB_ADD_PRESERVE : in this mode, old data are kept during insertion. - NCB_ADD_OVERWRT : new data will overwrite old ones. - NCB_ADD_COMPARE : this mode adds a new test in check stage. The overlapping old and new data must be identical or else the insertion is not conducted. An error NCB_RET_DATA_REJ is used in this case. The mode is specified with a new argument to ncb_add() function.