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const argv = require('yargs/yargs')(process.argv.slice(2)).command(  'math',  'math description',  yargs =>    yargs      .command(        'add <a> <b>',        'add description',        yargs =>          yargs            .positional('a', {              describe: 'addend "a"',              type: 'number',              default: 0,            })            .positional('b', {              describe: 'addend "b"',              type: 'number',              default: 0,            }),        argv => {          const {a, b} = argv;          console.log(`${a} + ${b} = ${a + b}`);        }      )      .command(        'sum <numbers..>',        'sum description',        yargs =>          yargs            .positional('numbers', {              describe: 'numbers to sum',              type: 'array',              default: [],            })            .check(argv =>              isArrayOfNumbers(argv.numbers)                ? true                : 'Positional argument "numbers" must only contain numbers'            ),        argv => {          const sum = argv.numbers.reduce((a, b) => a + b, 0);          console.log(`The sum of numbers is ${sum}`);        }      )).argv;
console.log(argv);
function isArrayOfNumbers(arr) {  return Array.isArray(arr) && arr.every(n => typeof n === 'number');}
// NOTE: ".argv" and ".parse()" should only be used at top level, not inside builder functions.