Hello guys, 

I'm just started learning python, so I need some pointers with a problem with lists. 

Suppose you have two lists

L1 = [2,3,2,5,7]


L2 = [2,2,2,3,5,11,13].

I want to extract only the elements of L2 that are NOT included already in L1 (including repetitions), i.e.  

L3 = [2,2,11,13]

(I repetead the 2 because L1 has two instance while L2 has three). 

I tried using sets but they remove the repetitions. The repetitions do matter for this problem.

Anyone know some particular technique to solve it? 
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