toHtml method
Implementation
String toHtml() {
return '<article id=${getId()}>'
'${Heading(title: "$number - $name ($abbreviation)").toHtml()}'
'${description.lines.map((e) => '<p>$e</p>').join()}'
'<section>'
'${Heading(title: "Focuses").toHtml()}'
'${focuses.map((e) => e.toHtml()).join()}'
'</section>'
'</article>';
}