When I am implementing fraud proofs in Bitcoin script, I find it useful to have an opcode "OP_SUCCESS" that will mark the execution to be successful without running the rest of the script, if this opcode is being executed. This is useful for writing code for fraud proofs such as BitVM, where the verifier wins if it finds one mismatch, and the verifier does not need to show the other mismatches.<div><br /></div><div>This OP_SUCCESS is weaker version of the OP_SUCCESSx in the Taproot upgrade, which marks the execution as successful for the mere presence of OP_SUCCESSx anywhere in the script. Rusty Russell in a 2023 article, "Covenants: Examining ScriptPubkeys in Bitcoin Script", also mentioned about the usefulness of such an opcode. </div><div><br /></div><div>Of course, this opcode can be emulated, and one can rewrite the existing script in a way to realize the same functionality without adding a new opcode to Bitcoin.</div><div><br /></div><div>The problem is that such rewriting is indeed fairly complicated. For example, say that we have the following program.</div><div><br /></div><div>```<br />OP_NOP1<br />OP_IF<br />    OP_NOP2<br />    OP_IF_SUCCESS<br />    OP_NOP3<br />    OP_IF_SUCCESS<br />    OP_NOP4<br />    OP_IF_SUCCESS<br />    OP_NOP5<br />OP_ENDIF<br />OP_NOP6<br />```</div><div>with OP_IF_SUCCESS short for "OP_IF OP_SUCCESS OP_ENDIF"</div><div><br /></div><div>The equivalent version without using new opcode is as follows (generated by computer, using a rewriting tool: https://github.com/Bitcoin-Wildlife-Sanctuary/fraud-proof-compiler)</div><div><br /></div><div>```</div><div>OP_NOP1<br />OP_IF<br />    OP_NOP2<br />    OP_IF<br />        1</div><div>        0</div><div>    OP_ELSE<br />        OP_NOP3</div><div>        OP_IF</div><div>            1</div><div>            0</div><div>        OP_ELSE</div><div>            OP_NOP4</div><div>            OP_IF</div><div>                1</div><div>            OP_ELSE</div><div>                OP_NOP5</div><div>                0</div><div>                0</div><div>                0</div><div>            OP_ENDIF</div><div>        OP_ENDIF</div><div>        OP_IF 1 OP_ENDIF</div><div>    OP_ENDIF<br />    OP_IF 1 OP_ENDIF<br />OP_ELSE<br />    0<br />OP_ENDIF<br />OP_IF<br />    1<br />OP_ELSE<br />    OP_NOP6<br />    0<br />OP_ENDIF<br />OP_IF</div><div>    OP_DEPTH 512 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (256 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 256 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (128 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 128 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (64 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 64 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (32 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 32 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (16 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 16 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (8 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 8 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (4 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 4 OP_GREATERTHANOREQUAL OP_IF<br />        OP_2DROP ... OP_2DROP (2 OP_2DROP)</div><div>    OP_ENDIF<br />    OP_DEPTH 2 OP_GREATERTHANOREQUAL OP_IF</div><div>        OP_2DROP</div><div>    OP_ENDIF<br />    OP_DEPTH OP_IF<br />        OP_DROP</div><div>    OP_ENDIF</div><div>    OP_TRUE</div><div>OP_ENDIF</div><div>```</div><div><br /></div><div>The second part of the code is mainly a general-purpose and stack-size-independent method to remove all the stack elements in order to be standard. </div><div><br /></div><div>For this reason, it seems better to avoid the need for developers to "emulate" such a weak version of OP_SUCCESS but to actually implement an opcode for that.</div><div><br /></div><div>What do you think?</div><div><br /></div>

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