enhance and better document PopReader()

    space_in_quoted_tokens = true;

    commentsAreTokens = false;

    limit = start;

    next  = start;

}

    readerStack.push_back( aLineReader );

    reader = aLineReader;

    start = (char*) (*aLineReader);

    // force a new readLine() as first thing.

    limit = start;

    next  = start;

}

void DSNLEXER::PopReader()

bool DSNLEXER::PopReader()

{

    // the very last reader cannot be popped, for that would screw up limit and next.

    if( readerStack.size() >= 2 )

    {

        readerStack.pop_back();

        reader = &readerStack.back();

    if( reader )

        start = (char*) (*reader);

        // force a new readLine() as first thing.

        limit = start;

        next  = start;

    }

}

    /**

     * Function PushReader

     * manages a stack of LINE_READERs in order to handle nested file inclusion.

     * Pushes aLineReader onto the top of a stack of LINE_READERs and makes

     * This function pushes aLineReader onto the top of a stack of LINE_READERs and makes

     * it the current LINE_READER with its own GetSource(), line number and line text.

     * A grammar must be designed such that the "include" token (whatever its various names),

     * and any of its parameters are not followed by anything on that same line,

     * because PopReader always starts reading from a new line upon returning to

     * the original LINE_READER.

     */

    void PushReader( LINE_READER* aLineReader );

     * in the case of FILE_LINE_READER this means the associated FILE is closed.

     * The most recently used former LINE_READER on the stack becomes the

     * current LINE_READER and its previous position in its input stream and the

     * its latest line number should pertain.

     * its latest line number should pertain.  PopReader always starts reading 

     * from a new line upon returning to the previous LINE_READER.  A pop is only

     * possible if there are at least 2 LINE_READERs on the stack, since popping

     * the last one is not supported.

     *

     * @return bool - true if there was at least two readers on the stack and 

     *  therefore the pop succeeded, else false and the pop failed.

     */

    void PopReader();

    bool PopReader();

    // Some functions whose return value is best overloaded to return an enum

    // in a derived class.