Like nailing jelly to a tree
/L-N-J-T-A-T/
adj. Used to describe a task thought to be impossible, esp. one in which the difficulty arises from poor specification or inherent slipperiness in the problem domain.
"Trying to display the 'prettiest' arrangement of nodes and arcs that diagrams a given graph is like nailing jelly to a tree, because nobody's sure what 'prettiest' means algorithmically."
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