@ Here's a fun way to count the number of bits in a tetrabyte.
[This classical trick is called the ``Gillies--Miller method
for sideways addition'' in {\sl The Preparation of Programs
for an Electronic Digital Computer\/} by Wilkes, Wheeler, and
Gill, second edition (Reading, Mass.:\ Addison--Wesley, 1957),
191--193. Some of the tricks used here were suggested by
Balbir Singh, Peter Rossmanith, and Stefan Schwoon.]
@^Gillies, Donald Bruce@>
@^Miller, Jeffrey Charles Percy@>
@^Wilkes, Maurice Vincent@>
@^Wheeler, David John@>
@^Gill, Stanley@>
@^Singh, Balbir@>
@^Rossmanith, Peter@>
@^Schwoon, Stefan@>

@<Subr...@>=
int count_bits @,@,@[ARGS((tetra))@];@+@t}\6{@>
int count_bits(x)
  tetra x;
{
  register int xx=x;
  xx=xx-((xx>>1)&0x55555555);
  xx=(xx&0x33333333)+((xx>>2)&0x33333333);
  xx=(xx+(xx>>4))&0x0f0f0f0f;
  xx=xx+(xx>>8);
  return (xx+(xx>>16)) & 0xff;
}