What is the line “大势一去水东流” from?
“大势一去水东流” seemingly means &quot;大势一去 就如長江水东流&quot; (when defeat becomes inevitable, it is like the water of Yangtze River flow East, --It can't be turned back)
Using &quot;River water run East&quot; metaphorically for the meaning of  &quot;irreversible situation&quot; is very common in Chinese poetry, the following are some examples:

《念奴娇 赤壁怀古》宋·苏轼：&quot;大江东去  .....&quot; (The water of Yangtze River flow East and won't return...)

~

《臨江仙》楊慎:
&quot;滚滚长江东逝水....&quot; (The water of the rolling Yangtze River disappear to the East...)

~

《虞美人》 南唐后主李煜:
&quot;....一江春水向東流&quot; (....The water of a river flow East in Spring)


“事其所以观其所由， came from 论语 “视其所以，观其所由，察其所安，人焉廋哉！&quot;

大势一去  is referring to the idiom 大势已去 宋·朱熹《朱子谏类》卷五十一：“.... 莫是大势已去。”

The phrase 水东流 appeared in the first line of 满江红·&quot;汉水东流....&quot;


The '戏'(opera) that 古老夫子 '唱'(sung) was a mix and match from different sources -- an original creation from the author of 《普提山下》陈果卿