Here are the grammar rules for the reverse polish notation calculator.
        verbinsert(RULES)(rpn/parser/grammar)
    The groupings of the tt(rpn) `language' defined here are the expression
(given the name tt(exp)), the line of input (tt(line)), and the complete input
transcript (tt(input)). Each of these nonterminal symbols has several
alternate rules, joined by the `tt(|)' punctuator which is read as the logical
em(or). The following sections explain what these rules mean.

The semantics of the language is determined by the actions taken when a
grouping is recognized. The actions are the bf(C++) code that appears inside
braces. See section ref(ACTIONS).

You must specify these actions in bf(C++), but b() provides the means for
passing semantic values between the rules. In each action, the pseudo-variable
tt($$) represents the semantic value for the grouping that the rule is going
to construct. Assigning a value to tt($$) is the main job of most actions. The
semantic values of the components of the rule are referred to as tt($1) (the
first component of a rule), tt($2) (the second component), and so on.