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Textual Positions of Tokens
---------------------------

   If you are using the `@N'-feature (Action Features) in actions
to keep track of the textual locations of tokens and groupings, then
you must provide this information in `yylex'.  The function `yyparse' 
expects to find the textual location of a token just parsed in the global
variable `yylloc'.  So `yylex' must store the proper data in that variable.
The value of `yylloc' is a structure and you need only initialize the
members that are going to be used by the actions.  The four members are
called `first_line', `first_column', `last_line' and `last_column'.
Note that the use of this feature makes the parser noticeably slower.

   The data type of `yylloc' has the name `YYLTYPE'.