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Subject: bijective association
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Hello,

Please, knows someone if there are any module who implements a bijective 
association ( 'a <=> 'b)?

functions should be something like (assuming 't is the type of the 
association):
add : 't -> 'a -> 'b -> 't
find_a  : 't -> 'a -> 'b
find_b : 't -> 'b -> 'a
remove : 't -> 'a -> 't
...

If not, which one could be the best option: use two associative lists, 
two maps, two hash tables or others?

Thanks a lot.