To elaborate, this is because of D's Uniform Function Call Syntax (http://www.drdobbs.com/blogs/cpp/232700394 http://www.drdobbs.com/blogs/cpp/232700394) - any

To elaborate, this is because of D's Uniform Function Call Syntax (http://www.drdobbs.com/blogs/cpp/232700394 http://www.drdobbs.com/blogs/cpp/232700394) - any function that takes an object as first argument can be called as though it were a method of that object; "map" and "filter" are actually functions in the std.algorithm module. It's a pretty neat trick and although in principle it could make things harder to reason about I haven't had any problems with it so far.