龙脉: A term used by geomancers. It refers to the good fengshui of a mountain chain with peaks that rise and fall unevenly
Originally referring to how the rise and fall of mountain peaks looks like dragon veins, this chengyu now generally refers to:
a) Someone or something's background
b) Someone or something's entire process of development from start to finish
Neutral
小明正在给大家讲述事件的来龙去脉 | Xiǎomíng zhèngzài gěi dàjiā jiǎngshù shìjiàn de láilóngqùmài
Little Ming is currently explaining the whole situation to everyone
[Source]
Python程序员需要花一点时间来熟悉生成器的来龙去脉 | Python chéngxùyuán xūyào huā yīdiǎn shíjiān lái shúxī shēngchéngqì de láilóngqùmài
It takes a little while for Python programmers to become familiar with the ins and outs of generators
[Source]
The ins and outs
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《运甓记》(Yùn Bì Jì), a Míng Dynasty (明, 1368 - 1644) work by 吾邱瑞 (Wú Quī Ruì)
This chengyu comes from a line in the work listed in the "Origin" section above