Syntax: bf(%scanner-token-function) tt(function-call) nl()

           The scanner function returning the next token, called from the
            generated parser's tt(lex) function. A complete function
            call expression should be provided (including a scanner object, if
            used). Example: 
            verb(
    %scanner-token-function d_scanner.lex()
                )
           If the function call contains white space
            tt(scanner-token-function) should be surrounded by double quotes.

It is an error if this directive is used and the specified scanner token
function is not called from the code in an already existing implementation
header.
