proc binOpAdd[T](x, y: T): T

iterator accumulate[T](iterable: openArray[T]; binop: BinOp[T] = binOpAdd;
                       inital = mayZeroDefault(T)): T

iterator combinations[T](iterable: openArray[T]; r: int): seq[T]

template mayZeroDefault[T](t: typedesc[T]): T