This case is tangled and knotty - hard to untangle.
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history
From 《后汉书·虞诩传》: Yu Xu, sent to a troubled post, says 不遇盘根错节，何以别利器乎？ - 'without encountering twisted roots and knotty joints, how would one know a sharp blade?' Originally a claim that hard cases prove capable officials; now mostly about the difficulty itself.
usage
Applied to investigations, bureaucracies, networks of corruption, long-running disputes - anywhere many threads interweave. Often pairs with 关系, 案件, 矛盾, 问题.
