PARABOL allows you to create a parabola by defining the
directrix, the focus, and the start point and endpoint for
the entity.

A parabola is a set of points in a plane, with the 
condition that the distance from any point in the set to 
a given line (i.e., the directrix) is equal to the 
distance from that point to a given point (i.e., the 
focus).

Define the directrix by choosing from the following 
options:

2 PTS  - to define the directrix by indicating two 
         positions as its endpoints.

LINE   - to define the directrix with an existing line.

ENDENT - to define the directrix as the line that is 
        tangent to a selected curve entity at the end of 
        the curve that is closest to the selection point.
