makes accumulate iters with lambda assignable
makes accumulate iters with lambda assignable

tempbottle/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools