<https://ieltstutorials.online/blog/know-the-difference-between-formal-and-informal-english-for-letter-writing-in-ielts-test>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://ieltstutorials.online/blog/know-the-difference-between-formal-and-informal-english-for-letter-writing-in-ielts-test>