old_contents: from __future__ import absolute_import
  
  from .block import Division, Paragraph, Preformatted
  from .document import (Document, HTMLRoot, Head, Body, Title, Meta, Script,
                         HeadLink, Main)
  from .element import (Element, VoidElement,
                        html_attribute, boolean_html_attribute,
                        int_html_attribute)
  from .generator import (Generator, NullGenerator, ChildGenerator,
                          HTMLChildGenerator, JoinGenerator, HTMLJoinGenerator)
  from .image import Image
  from .inline import Span, Highlight, Strong, Alternate, Emphasis, Small
  from .link import Link
  from .list import (OrderedList, UnorderedList, ListItem, DescriptionList,
                     DescriptionDefinition, DescriptionTerm)
  from .structure import (Section, Article, Navigation, Aside, Header, Footer,
                          Heading)
  from .table import (Table, TableHead, TableBody, TableRow, TableCell,
                      ColumnGroup, Column)
  from .time import Time
  
new_contents: from __future__ import absolute_import
  
  from .block import Division, Paragraph, Preformatted
  from .document import (Document, HTMLRoot, Head, Body, Title, Meta, Script,
                         HeadLink, Main)
  from .element import (Element, VoidElement,
                        html_attribute, boolean_html_attribute,
                        int_html_attribute)
  from .generator import (Generator, NullGenerator, ChildGenerator,
                          HTMLChildGenerator, JoinGenerator, HTMLJoinGenerator)
  from .image import Image
  from .inline import Span, Highlight, Strong, Alternate, Emphasis, Small
  from .link import Link
  from .list import (OrderedList, UnorderedList, ListItem, DescriptionList,
                     DescriptionDefinition, DescriptionTerm)
  from .structure import (Section, Article, Navigation, Aside, Header, Footer,
                          Heading)
  from .table import (Table, TableHead, TableBody, TableRow, TableHeaderCell,
                      TableCell,
                      ColumnGroup, Column)
  from .time import Time
  
subject: add TableHeaderCell to htmlgen