The grade :top corresponds to the nonterminal expression;
the grade :ior corresponds to
the nonterminal inclusive-OR-expression;
the grade :xor corresponds to
the nonterminal exclusive-OR-expression;
each of the other grades, :<grade>, corresponds to
the nonterminal <grade>-expression.
We stop at primary expressions:
we do not need grades for identifier, constant, etc.
