lookahead on ranges

[Sebastian Graf]

Hi,

is there any way to to something like

     auto arr = [1,2,3,4,5];
     auto delta = arr.lookahead!"b-a"(1); // or probably pass 1 as 
template arg
     assert(equal(delta[], [1,1,1,1][]);

or like

     // lookahead returns range of tuples (template arg) or arrays 
(runtime arg)
     foreach (a, b; arr.lookahead!1)
         writeln(b-a);

on a range? I think I could possibly do this with zip, but I am 
curious if there is something more to the point to do it in 
phobos. In particular something that just buffers n eles and does 
not copy the input range to popFront() all of them.