When you use %union to specify multiple value types, you must
declare the value type of each nonterminal symbol for which values are
used. This is done with a %type declaration, like this:
%type <type> nonterminal...
Here nonterminal is the name of a nonterminal symbol, and type
is the name given in the %union to the alternative that you want
(see section The Collection of Value Types). You can give any number of nonterminal symbols in
the same %type declaration, if they have the same value type. Use
spaces to separate the symbol names.