+** Types of values for %define variables
+
+ Bison used to make no difference between '%define foo bar' and '%define
+ foo "bar"'. The former is now called a 'keyword value', and the latter a
+ 'string value'. A third kind was added: 'code values', such as '%define
+ foo {bar}'.
+
+ Keyword variables are used for fixed value sets, e.g.,
+
+ %define lr.type lalr
+
+ Code variables are used for value in the target language, e.g.,
+
+ %define api.value.type {struct semantic_type}
+
+ String variables are used remaining cases, e.g. file names.
+