sub cell {
  # $tab is a number.
  # $type is either 'h' for heading or 'd' for data.
  # $contents is the text in the cell.
  # $opt is all of the available options to be adding inside the opening tag.
  my ($tab,$type,$contents,$opt) = @_;

  my @attributes;
  push @attributes, qq(id="$opt->{id}")       if $opt->{id};
  push @attributes, qq(class="$opt->{class}") if $opt->{class};
  push @attributes, qq(style="$opt->{style}") if $opt->{style};
  push @attributes, qq(colspan="$opt->{colspan}") if $opt->{colspan};
  push @attributes, qq(rowspan="$opt->{rowspan}") if $opt->{rowspan};

  my $format = @attributes > 0 ? ' '.join(' ',@attributes) : '';
  my $open = $type.$format;
  
  line($tab,qq(<t$open>));
  # Lists are treated specially in cells. The list subroutine is below.
  if ($contents =~ /list/i) {
    if (ref($opt->{list}[1]) eq 'HASH') {
      list($tab + 1,$opt->{list}->[0],$opt->{list}->[1]);
    }
    else {
      list($tab + 1,$opt->{list});
    }
  }
  else {
    line($tab + 1,$contents);
  }
  line($tab,qq(</t$type>));
}