On the second thought, maybe checking for overflows after each 64-bit arithmetic opcode is not that great, if we can have computations in (practically) arbitrary width integers and then only detect overflows on conversion to LE64/LE32.
(but really, adding VERIFY after each 64-bit arith op does not add any mental load - it just increases script size a bit)
(well, forgetting VERIFY after 64-bit arith op in Elements might lead to unexpected behavior in case of actual overflow…)