Fix in the compiler (could loop on circular structures)
Compiling (cons #0=(a . #0#) #1=(a . #1#)) lead to an infinite loop since it searches in contants using the member function . Using memq solves the problem, at the expense of a little bigger tables. Using (member x y equiv?) is not a good idea (until it is optimized) since each bootstrap phase is more than 10 times slower.
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