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The code for the cardinality method for elliptic curves can be found on the file src/sage/schemes/elliptic_curves/ (you can click to see it). As you can see (search for def cardinality), the default implementation is by calling pari, so you can have a look to its source code (in the C language) and try to make it early-aborting. There is an alternative (probably slower) implementation in Sage, which is called cardinality_bsgs, so you can have a look at it and see if you can deduce that the cardinality is not prime before reaching return self._order commands.