Then the parse tree of those before will be constructed before
those latter.
In your example, item_declaration will be constructed before
item_list, so
one can have actions
item_declaration {...} item_list {...}
Here, the first action {...} can only use the item_declaration
value, but
the second can use all that comes before. - See the Bison manual for
details.
At the end of that rule, the value of item_declaration is $1 and
item_list
is $2 and the overall return value is $$. What would be the value of
item_declaration in my action if I put the latter just straight
after it
(not at the end of the rule)?