def ramp10(params, phase, args=dict(n=2, guess=[1, 0.2])):
    """Model Ramp Eq. 10 from Stevenson et al. (2011).

    params: 2-sequence
      parameters that define the function, as shown below.

    phase: NumPy array. 
      Orbital phase (or more generally, 'time')

    Functional form:
        ramp = r[0] * (1. + r[1] * (phase - 0.5))
    """
    # 2013-12-07 14:08 IJMC: Created.

    return params[0] * (1. + params[1] * (phase - 0.5))