# Combining Stack Queries The real power of stack queries emerges when combining **OP\_DEPTH** and **OP\_SIZE** with conditional logic to create sophisticated validation patterns. ### Example: Adaptive Payment Script ``` OP_DEPTH OP_1 OP_EQUAL OP_IF // Single input: validate it's a 65-byte signature OP_SIZE OP_65 OP_EQUAL OP_VERIFY OP_CHECKSIG OP_ELSE // Multiple inputs: validate format and check multisig OP_SIZE OP_33 OP_EQUAL OP_VERIFY // Verify pubkey size OP_SWAP OP_SIZE OP_65 OP_EQUAL OP_VERIFY // Verify signature size OP_CHECKSIG OP_ENDIF ``` **What this achieves:** 1. **Depth check:** Determines input pattern (signature-only vs pubkey+signature) 2. **Size validation:** Ensures each input meets expected byte length 3. **Format enforcement:** Rejects malformed data before cryptographic operations 4. **Signature verification:** Checks authentication after validation **Why combine queries:** * **Layered validation:** Check structure, then format, then content * **Early failure:** Reject invalid inputs before expensive operations * **Clear error points:** Each check identifies specific validation failure * **Efficiency:** Avoid unnecessary computation on malformed data
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