Why is there both a ForwardOp and BackwardOp template argument to iter_fold_if? https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/iter_fold_if.hpp#L6... I would have thought that only 1 was needed. Since iter_fold_if is not documented here: http://www.boost.org/doc/libs/1_38_0/libs/mpl/doc/refmanual/refmanual_toc.ht... I assume it's not meant to be used by the user, but is only an implementation detail; however, it, or a variation, might be useful to implement the and_seq mentioned here: http://article.gmane.org/gmane.comp.lib.boost.devel/186204 As shown in that post, the template, while_if_then was used, and it does not require anything like both a ForwardOp and BackwardOp, it just requires the If and Then unary metafunctions. One problem I encountered with while_if_then was moving to the end of the sequence, at which time the Then metafunction couldn't be called; hence, I had to make it lazy to avoid dereferencing the end iterator. Is that somehow related to the need for BackwardOp in iter_fold_if. IOW, because iter_fold_if might reach the end iterator, it has to backup? I'm really asking because in the variadic template version of mpl, I wondering if iter_fold_if could be replaced with something like while_if_then (which may be renamed fold_if to be more consistent with other names with similar function). Any insight about the rationale for the iter_fold_if would probably save me considerable time in deciding whether it can be replaced with the proposed fold_if. -regards, Larry