difference and repetition

https://tang.skidmore.edu/index.php/calendars/view/139/tag:1/year:all

https://www.youtube.com/watch?v=EdmMM6iAt8E&list=PLoHSL9k934uvazlrXmntGNAWKp7ZT95sx&index=2

https://www.youtube.com/watch?v=0VQVhkDDf14&list=PLoHSL9k934uvazlrXmntGNAWKp7ZT95sx&index=11