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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def install(pkg,
            dir,
            pkgs=None,
            runas=None,
            env=None):
    '''
    Install a Bower package.

    If no package is specified, the dependencies (from bower.json) of the
    package in the given directory will be installed.

    pkg
        A package name in any format accepted by Bower, including a version
        identifier

    dir
        The target directory in which to install the package

    pkgs
        A list of package names in the same format as the ``pkg`` parameter

    runas
        The user to run Bower with

    env
        Environment variables to set when invoking Bower. Uses the same ``env``
        format as the :py:func:`cmd.run <salt.modules.cmdmod.run>` execution
        function.

    CLI Example:

    .. code-block:: bash

        salt '*' bower.install underscore /path/to/project

        salt '*' bower.install jquery#2.0 /path/to/project

    '''
    _check_valid_version()

    cmd = _construct_bower_command('install')

    if pkg:
        cmd.append(pkg)
    elif pkgs:
        cmd.extend(pkgs)

    result = __salt__['cmd.run_all'](cmd,
                                     cwd=dir,
                                     runas=runas,
                                     env=env,
                                     python_shell=False)

    if result['retcode'] != 0:
        raise CommandExecutionError(result['stderr'])

    # If package is already installed, Bower will emit empty dict to STDOUT
    stdout = salt.utils.json.loads(result['stdout'])
    return stdout != {}