sub update_thing
{
  my ($self, $x, $y, $u) = @_;
  if (ref $u and UNIVERSAL::isa($u, 'CODE'))
  {
    # use temp to keep the $u function from changing $_[0] ;-)
    my $val = $self->{cell}[$x][$y];
    $u = $u->($val);
  }
  # validate $u.
  if ($is_valid)
  {
    $self->{cell}[$x][$y] = $u;
}