[boost] [proto] run-time error

Eric, the attached file exibits a problem with (I think) display_expr: The second line in main() fails. But if you comment out the definition of META_EXPR(plus,left,...) and replace it with the one just below it and presently commented out then everything is fine. Seems like display_expr is not appy with function applied to more than one argument. Unless there's something stupid in my code. Regards, Maurizio