The diagram shows a parabola that is symmetrical about the -axis and whose vertex is at
the origin. It can be described by the parametric equations and , , where is a positive constant. The focus of the parabola is the point
, and the directrix is the line with the equation .

Find a pair of parametric equations that describe the parabola whose focus is the point
and whose directrix is the line . Include the parameter
range.