sub is_same_leading_bit {
  my ($v1, $v2) = (shift, shift);
  my $and = 1 << 63;
  for my $shift(0 .. 63) {
    my $c1 = ($v1 << $shift) & $and;
    my $c2 = ($v2 << $shift) & $and;
    return 1 if $c1 & $c2;
    return 0 if $c1 | $c2;
  }

  return 0; # both args are 0.
}