interface SigVerifier {

    /// @dev Verifies that a signature is valid.
    /// @param hash Message hash that is signed.
    /// @param signature Proof of signing.
    /// @return Validity of order signature.
    function isValidSignature(
        bytes32 hash,
        bytes signature
    )
        external
        view
        returns (bool isValid);
}
function isValidSignature(
    bytes32 hash,
    bytes signature
)
    external
    view
    returns (bool isValid);

/// @dev Verifies that a signature is valid.
/// @param hash Message hash that is signed.
/// @param signature Proof of signing.
/// @return Validity of order signature.