my $b = 7 ;              # Number of bits to worry about
  my $p = ($b + 1) >> 1 ;  # Number of pairs of bits to consider
  my $cnt = 0 ;
  for my $v (0..((2**$b)-1)) {
    my $m = 0b11 ;
    for (1..$p) {
      if ($v & $m) { $m <<= 2 ;      }
              else { $cnt++ ; last ; } ;
    } ;
  } ;