(require) means that the line is required for any Solidity source file while (optional) indicates that the line is not always needed. The symbol Ln: is not part of the Solidity code and is included here only to show the line numbers. Please do not include these symbols in source code intended for real use.//) are comments rather than code; they are used to annotate and explain the code. The compiler ignores comments.pragma statement in L01 indicates the minimum compiler version. - The import statement in L03 imports all global symbols from "filename". filename should be an actual file name.L05 - L20 define a smart contract called UserStorage. The keyword contract is located before the contract name and declares that the code represents a smart contract. Contracts in Solidity are similar to classes in object-oriented languages. Each contract can contain declarations for state variables, functions, function modifiers, events, struct types and enum types. Furthermore, contracts can inherit from other contracts. The example code contains one contract definition, but a single Solidity file may contain more than one contract definition.L07, userData is a state variable for the mapping type. State variables are permanently stored in contract storage. The state variable userData maintains a mapping between address and a uint value. The address type holds a 20-byte address (Klaytn uses a 20-byte address similar to Ethereum).L09 defines a public function set that saves the value x in userData for the message's sender. The variable msg.sender is a special variable defined in Solidity that represents the address of the message (i.e., current call) sender. The keyword public means that the function is part of the contract interface and can be called externally or internally.get in L13 and getUserData in L17 are declared with view, which means that the functions promise not to modify any state variable. Their declarations include returns (uint), which implies that they return a uint value.UserStorage.sol (L03 is excluded in the source file shown above), some examples of compiling the file UserStorage.sol are as follows.--bin), an abstract syntax tree (by --ast), and assembly code (by --asm) as separate files in the output directory.--optimize flag.