<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_cartesian(r, theta, theta_units="radians"):
 """ Converts polar r, theta to cartesian x, y. """

    assert theta_units in ['radians', 'degrees'],\
        "kwarg theta_units must specified in radians or degrees"

    # Convert to radians
    if theta_units == "degrees":
        theta = to_radians(theta)

    theta = to_proper_radians(theta)
    x = r * cos(theta)
    y = r * sin(theta)

    return x, y