public int Arena()
{
var ar = new Arena <DisableBoundsCheck, NonZeroMemory>(new Span <byte>(buffer, size));
var men = ar.AllocSpan <Man>(N);
var elves = ar.AllocSpan <Elf>(N);
var dwarves = ar.AllocSpan <Dwarf>(N);
return(men.Length + elves.Length + dwarves.Length);
}