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The Lexical Analyzer Function `yylex'

   The "lexical analyzer" function, `yylex', recognizes tokens from the
input stream and returns them to the parser.  Bison does not create
this function automatically; you must write it so that `yyparse' can
call it.  The function is sometimes referred to as a lexical scanner.

   In simple programs, `yylex' is often defined at the end of the Bison
grammar file.  If `yylex' is defined in a separate source file, you
need to arrange for the token-type macro definitions to be available
there.  To do this, use the `-d' option when you run Bison, so that it
will write these macro definitions into a separate header file
`NAME.tab.h' which you can include in the other source files that need
it.  Note: Invoking Bison.

* Calling Convention
How `yyparse' calls `yylex'.
* Token Values
How `yylex' must return the semantic value of the token it has read.
* Token Positions
How `yylex' must return the text position (line number, etc.) of the token, if the actions want that.
* Pure Calling
How the calling convention differs in a pure parser (Note: A Pure (Reentrant) Parser.).

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