// Lets say we have a class called 'AParser' that contains all the methods to parse
    // your BNF.  The end result of parsing is one or more errors, OR a 'parse tree', which
    // is the internal representation of the language in tree form.  So, the parser's
    // job is to build the tree and detect errors.
    // Each production from the BNF can be represented as a single method.
    // The result of that method is an error or a node filled in for the caller to
    // do whatever it needs to (generally adding it to HER node and passing that
    // up the call chain too).
bool AParser::Number( AParseTreeNode& node )
{
        // The result of this routine will be to fill in 'node' or
        // give an error.  For working, we need to pass another
        // node to routines we call....
    AParseTreeNode childNode;

        // Set up the passed-in node with info about who I am for later reference
        // and/or error messages:
        node.SetType( kNumber );

        // And you may want to know later what source line and col this node came
        // from (for debugging output, say, or setting breakpoints, or whatnot)
    SetLineNumberInfoIntoNode( node );

        // Now, from the BNF, we know that a number is either an int or a real:
    if (Integer( childNode ) || Real( childNode ))
    {
        // Here we glue the child node onto our node, or maybe sometimes you might
        // morph the child node INTO our node, or whatever, depending on your needs.
        node.AddChild( childNode ); // makes a copy of childNode
    }
    else
    {
            // This might display the source line and column, too
        Error("Expected a NUMBER here.");
        return false;
    }
    return true;
}