Draws a circle or ellipse on the screen.
 
CIRCLE [STEP] (x!,y!),radius![,[color%] [,[start!] [,[end!] [,aspect!]]]]
 
    - STEP       Specifies that coordinates are relative to the current
                 graphics cursor position.
    - (x!,y!)    The coordinates for the center of the circle or ellipse.
    - radius!    The radius of the circle or ellipse in the units of the
                 current coordinate system, determined by the most recent
                 SCREEN, VIEW, and WINDOW statements.
    - color%     A color attribute that sets the circle's color. The
                 available color attributes depend on your graphics adapter
                 and the screen mode set by the most recent SCREEN statement.
    - start!     The starting angle for the arc, in radians.
    - end!       The ending angle for the arc, in radians.
    - aspect!    The ratio of the length of the y axis to the length of the
                 x axis, used to draw ellipses.
 
    - To convert from degrees to radians, multiply degrees by (PI / 180).
 
Example:
    'This example requires a color graphics adapter.
    SCREEN 2
    CIRCLE (320, 100), 200
    CIRCLE STEP (0,0), 100
 
See Also    <COLOR>    <DRAW>    <LINE>    <SCREEN>    <VIEW>    <WINDOW>
            <COLOR ATTRIBUTES AND VALUES>    <SCREEN MODES>