Macro argument function
@ARG(symbol | expression)
Returns integer 1 if the macro argument represented by symbol or expression is present, 0 otherwise. If the argument is a symbol, it must be single-quoted and refer to a dummy argument name. If the argument is an expression, it refers to the ordinal position of the argument in the macro dummy argument list. A warning is issued if this function is used when no macro expansion is active.
IF @ARG(TWIDDLE) ; twiddle factor provided?