+=head2 C<< vshift $v, $start, $length => $bits [, $insert ] >>
+
+In the area starting at C<$start> and of length C<$length> in C<$v>, shift bits C<abs $bits> positions left if C<< $bits > 0 >> and right otherwise.
+If C<$insert> is defined, also fills the resulting gap with ones if C<$insert> is true and zeros if it's false.
+Bits outside of the specified area are left untouched.
+Doesn't need to allocate any extra memory.
+
+=cut
+
+sub vshift {
+ my ($start, $length, $bits, $insert) = @_[1 .. 4];
+ return unless $bits;
+ my $left = 1;
+ if ($bits < 0) {
+ $left = 0;
+ $bits = -$bits;
+ }
+ $bits = $length if $bits > $length;
+ $length -= $bits;
+ if ($left) {
+ vcopy($_[0], $start, $_[0], $start + $bits, $length);
+ vfill($_[0], $start, $bits, $insert) if defined $insert;
+ } else {
+ vcopy($_[0], $start + $bits, $_[0], $start, $length);
+ vfill($_[0], $start + $length, $bits, $insert) if defined $insert;
+ }
+}
+