TAG_idx is the position where (for any abstract grammar tree) the tag of the tree resides. So, given any grammar tree, t, then t[TAG_idx] is a number that identifies the particular kind of tree we are dealing with. The result is a number that can be tested to be equal to NBR_tag,FRC_tag,TXT_tag, TAB_tag, FUN_tag,NAT_tag,VAR_tag, APL_tag, TBL_tag, DEF_tag,SET_tag,DCT_tag or VOI_tag.