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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 ci_macos():
 """ Setup Travis-CI macOS for wheel building """

    run_command("brew install $PYTHON pipenv || echo \"Installed PipEnv\"")
    command_string = "sudo -H $PIP install "
    for element in DEPENDENCIES + REQUIREMENTS + ["-U"]:
        command_string += element + " "
    run_command(command_string)
    # Build a wheel
    run_command("sudo -H $PYTHON setup.py bdist_wheel")
    assert check_wheel_existence()
    exit(0)