def move_forward(num_steps):
    """Moves the pen forward a few steps in the direction that its "turtle" is facing.

    Arguments:
        num_steps - a number like 20. A bigger number makes the pen move farther.
    """
    assert int(num_steps) == num_steps, "move_forward() only accepts integers, but you gave it " + str(num_steps)

    _make_cnc_request("move.forward./" + str(num_steps))

    state['turtle'].forward(num_steps)