table_heading_tags
  = "!"
    (
        table_heading_tag_no_nl<th>
        / (
            table_heading_tag
            (
                ("!!" / pipe_pipe)
                table_heading_tag
            )*
        )
    )
    (
        ("!!" / pipe_pipe)
        table_heading_tag_no_nl<th>
    )*
    (
        ("!!" / pipe_pipe)
        table_heading_tag
    )*

table_heading_tag_no_nl
  = table_heading_tag
    & {
        return text().indexOf("\n") == -1;
    }