Hi! I want to discuss the internal data structure expr<>: I like the way expr<> is built regarding to the first and second template argument: It is well thought and done to have the expression tag as first argument and some fusion-compatible typelist as its second argument containing the operands. OTOH the third template argument contains redundant information: The docs say: "Proto expression nodes are valid Fusion random-access sequences of their children nodes." So the information about the number of arguments is accessible via some size<T>::type metafunction e.g. depending on result_of::distance<result_of::begin<S>::type result_of::end<S>::type>::type? In my unfinished attempts to write a paper about ET library design (now obsolote, thanks to Eric) I have a statement that any expression representation taking more than 2 template arguments is wrong. Eric, since you always have a good reason for your design: Could you please elaborate on this? Also I ask myself whether there was a good reason not to follow mpl and use types to represent numbers. I'd prefer at least some parallism to mpl here: size_t<c>? Markus