old_contents: #!/usr/bin/env node
  
  const yargs = require('yargs');
  
  yargs
    .usage('react-power <cmd> [args]')
    .command(
      'dev',
      'Run in development environment.',
      () => {},
      argv => {
        require('../lib/dev');
      }
    )
    .command(
      'build',
      'Run in production environment.',
      yargs => yargs.option('watch', {
        alias: 'w',
        default: false
      }),
      argv => {
        process._argv = argv
        require('../lib/build');
      }
    )
    .command(
      'clean',
      'Clean the work directory.',
      () => {},
      argv => {
        require('../lib/clean');
      }
    )
    .command('*', 'the default command.',
      () => {},
      argv => {
      console.log('Command not found. Run `react-power --help` for usage.')
    })
    .help().argv;
  
new_contents: #!/usr/bin/env node
  
  const yargs = require('yargs');
  
  yargs
    .usage('react-power <cmd> [args]')
    .command(
      'dev',
      'Run in development environment.',
      () => {},
      argv => {
        require('../lib/dev');
      }
    )
    .command(
      'build',
      'Run in production environment.',
      yargs => yargs.option('watch', {
        alias: 'w',
        default: false
      }),
      argv => {
        process._argv = argv
        require('../lib/clean');
        require('../lib/build');
      }
    )
    .command(
      'clean',
      'Clean the work directory.',
      () => {},
      argv => {
        require('../lib/clean');
      }
    )
    .command(
      '*',
      'The default command.',
      () => {},
      argv => {
      console.log('Command not found. Run `react-power --help` for usage.')
    })
    .help().argv;
  
subject: clean the dist directory when before build