You could define ENABLE64BIT to result in script failure unless it is the first opcode (apart from any other OP_SUCCESSx ops – that way future upgrades that introduced ENABLExyz opcodes could then relax that restriction to allow ENABLE* opcodes to be in any order at the start of the script). You could have that requirement enforced even if ENABLE64BIT occurred in an unexecuted IF/ELSE branch.
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