If the objects pointed to by *p1 and *p2 have the same type, increment their reference count and return 0 (success). If the objects can be converted to a common numeric type, replace *p1 and *p2 by their converted value (with 'new' reference counts), and return 0. If no conversion is possible, or if some other error occurs, return -1 (failure) and don't increment the reference counts. The call PyNumber_Coerce(&o1, &o2) is equivalent to the Python statement "o1, o2 = coerce(o1, o2)".
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