Why are the substring opcodes disabled? I understand that OP_CAT is disabled because it can blow up the stack easily (in combination with OP_DUP). But the substring opcodes reduce the stack size.
I'd like to use OP_LEFT to implement a proof of work requirement to spend a transaction. A substring operation is needed because OP_ADD (for salting) and OP_GREATERTHAN only work on 32bit values.