... let _singleton := sload(0)
// 0xa619486e == keccak("masterCopy()"). The value is right padded to 32-bytes with 0s
if eq(calldataload(0), 0xa619486e00000000000000000000000000000000000000000000000000000000) {
@> mstore(0, shr(12, shl(12, _singleton)))
...