NAME:
    cgetrng
 PURPOSE: (one line)
    How to integrate over a circle.
 DESCRIPTION:
    This procedure is called to determine how to iterate when integrating
 over a circle.  The circle's center is at (xc,yc), and its radius is r.
 For pixels with x-coordinate x, those in the intervals [y0,y1) and [y2,y3)
 are on or near the circle.  Those in the interval [y1,y2) are definitely
 inside; all others are definitely outside.
    Of course, the routine can be called to determine an interval for fixed
 y by calling it as cgetrng,yc,xc,r,y,x0,x1,x2,x3.
    The appropriate way to integrate over a circle is therefore as follows:
 cgetrng,xc,yc,r,Round(xc),y0,y1,y2,y3
 for (y = y0; y <= y3-1; y=y+1) {
    cgetrng, yc, xc, r, y, x0, x1, x2, x3;
    for (x = x0; x <= x1-1; x=x+1) sum = sum + value(x,y)*pixwt(xc,yc,r,x,y);
    for (x = x1; x <= x2-1; x=x+1) sum = sum + value(x,y);
    for (x = x2; x <= x3-1; x=x+1) sum = sum + value(x,y)*pixwt(xc,yc,r,x,y);
    }
 CATEGORY:
    CCD data processing
 CALLING SEQUENCE:
    cgetrng, xc, yc, r, x, y0, y1, y2, y3
 INPUTS:
    xc, yc : Center of the circle.
    r      : Radius of the circle.
    x      : X coordinate for the intervals to be determined.
 OPTIONAL INPUT PARAMETERS:
    None.
 KEYWORD PARAMETERS:
    None.
 OUTPUTS:
    y0, y1, y2, y3 : The endpoints of the three intervals of interest.
 COMMON BLOCKS:
    None.
 SIDE EFFECTS:
 RESTRICTIONS:
    None.
 PROCEDURE:
    Determine three intervals along the x input coordinate:  The
    intervals [y0,y1), [y1,y2), and [y2,y3).
 MODIFICATION HISTORY:
    Ported by Doug Loucks, Lowell Observatory, 1992 Sep, from the
    routine cgetrng in pixwt.c, by Marc Buie.
    4/1/93, DWL, Added argument validation (badpar).