Here are the bf(C++) and b() directives for the reverse polish notation
calculator. As in bf(C++), end-of-line comments may be used.
        verbinsert(//DECL)(rpn/parser/grammar)
    The directive section provides information to b() about the token
types (see section The b() Declarations Section). Each terminal symbol
that is not a single-character literal must be declared here (Single-character
literals normally don't need to be declared). In this example, all the
arithmetic operators are designated by single-character literals, so the only
terminal symbol that needs to be declared is tt(NUM), the token type for
numeric constants. Since b() uses the type tt(int) as the default
semantic value type, one additional directive is required to inform b()
about the fact that we are using tt(double) values. The tt(%stype)
(semantic value type) directive is used to do so.




