Trying to make FreeList Allocator example compile

[Per Nordlöw]

I'm trying to compile the example

     import std.experimental.allocator.building_blocks.free_list : 
FreeList;
     theAllocator = allocatorObject(FreeList!8());

at https://dlang.org/phobos/std_experimental_allocator.html but 
fails first because of missing import

     import std.experimental.allocator : allocatorObject, 
theAllocator;

and then, when adding this import, it fails again with the error

/home/per/Work/knet/phobos-next/snippets/gctester.d(127,40): 
Error: template instance `FreeList!8` does not match template 
declaration `FreeList(ParentAllocator, ulong minSize, ulong 
maxSize = minSize, Flag adaptive = No.adaptive)`

Why is this example missing the `ParentAllocator` template 
parameter and what is the preferred type for `ParentAllocator`?