Add function locked(...)

EIPS/eip-5192.md

@@ -29,14 +29,21 @@ The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SH
 
 Upon minting a token implementing this EIP, it must be permanently inseparable from the receiving account. All [EIP-721](./eip-721.md) functions of the contract that transfer the token from one account to another must throw.
 
-To aid recognition that an [EIP-721](./eip-721.md) token implements soulbinding via this EIP upon calling [EIP-721](./eip-721.md)'s `function supportsInterface(bytes4 interfaceID) external view returns (bool)` with EIP-5192's `SOULBOUND_VALUE=0x9e7d7f8` as `bytes4 interfaceID`, a contract implementing this EIP must return `true`.
-
-We compute `SOULBOUND_VALUE` by running the string "soulbound" through `keccak256` and casting it to a `bytes4`:
-
 ```solidity
-bytes4 constant SOULBOUND_VALUE = bytes4(keccak256("soulbound")); // 0x9e7ed7f8;
+// SPDX-License-Identifier: CC0-1.0
+pragma solidity ^0.8.0;
+
+interface IERC5192 {
+  /// @notice Returns the locking status of an Soulbound Token
+  /// @dev SBTs assigned to zero address are considered invalid, and querie
+  /// about them do throw.
+  /// @param tokenId The identifier for an SBT.
+  function locked(uint256 tokenId) external view returns (bool);
+}
 ```
 
+To aid recognition that an [EIP-721](./eip-721.md) token implements soulbinding via this EIP upon calling [EIP-721](./eip-721.md)'s `function supportsInterface(bytes4 interfaceID) external view returns (bool)` with `interfaceID=0xb45a3c0e`, a contract implementing this EIP must return `true`.
+
 ## Rationale
 
 The above model is the simplest possible path towards a canonical interface for Soulbound tokens. It reflects upon the numerous Soulbound token implementations that simply revert upon transfers.