TAB tag denotes the kind
of trees that is used to represent table values (notice the difference with
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.
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).