Question: how to output all chains with list of list?

chosen1 := [[1,2],[1,20],[3,4]]:
chosen2 := [[2,3],[20,3],[3,4]]:
chosen3 := [[3,4],[5,7]]:
chosen4 := [[4,5],[5,6]]:
chosen5 := [[5,6],[7,9]]:
 
hashtable can not work 
 
for example
[1,2]'s 2 as key to find 3 in chosen2 [2,3] then use 3 to find 4 in [3,4] then use 4 to find 5 in [4,5]
 
 
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