# -- PSEUDOCODE, DOES NOT WORK --
my ($dx, $dy);
sub mobile_start {
  my $ev = $zinc->XEvent;
  ($dx, $dy) = $zinc->transform(1, $scaled_group, [$ev->x, $ev->y]);
}

sub mobile_move {
  my $ev = $zinc->XEvent;
  my ($tr_evx, $tr_evy) = $zinc->transform(1, $scaled_group, [$ev->x, $ev->y]);
  $zinc->translate($some_group, $tr_evx-$dx, $tr_evy-$dy);
  ($dx, $dy) = ($tr_evx, $tr_evy);
}