Accumulator probably useful beyond its initial purpose with few modifications

[e_r]

In ​news://news.gmane.org:119/goeic6$q4t$1@ger.gmane.org

I mentioned that I found that much of the structure of Accumulators could be used for more general purposes involving dependent features.

In my own modified version, amongst other things, I dropped the dependency on RealType and the new definition of visitor is

visitor::operator()(feature) calls f(feature,args), where f and args are passed in the constructor of visitor.