This TAB tag denotes the kind of trees that is used to represent table values (notice the difference with TBL!!). If tt is such a table tree, then we can access its 'real underlying table' by using the expression tt[TAB_TAB_idx]. The result of this expression is a real Pico table that we can index using the normal indexing rules. The TAB tag is used at each place where one needs a grammar component that varies in its number of children. Examples are the formal and actual parameters of a function (call/definition).