like nailing jelly to a tree

like nailing jelly to a tree 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."