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     The _pie functions draw a pie-shaped wedge by drawing an
     elliptical arc whose center and two endpoints are joined by lines.
 
     The _pie function uses the view coordinate system. The center of
     the arc is the center of the bounding rectangle specified by the
     view-coordinate points (<x1>, <y1>) and (<x2>, <y2>). The arc
     starts where it intersects the vector defined by (<x3>, <y3>) and
     ends where it intersects the vector (<x4>, <y4>).
 
     The _pie_wxy routine (implemented as a macro) uses the window
     coordinate system. The center of the arc is the center of the
     bounding rectangle specified by the window-coordinate pairs
     (<pwxy1>, <pwxy2>). The arc starts where it intersects the vector
     defined by <pwxy3> and ends where it intersects the vector defined
     by <pwxy4>.
 
     The pie-shaped wedge is drawn in a counterclockwise direction,
     using the current color. The <control> parameter can be one of the
     following manifest constants: _GBORDER or _GFILLINTERIOR.
 
     The control option given by _GFILLINTERIOR is equivalent to a
     subsequent call to the _floodfill function using the center of the
     ellipse as the starting point and the current color (set by
     _setcolor) as the boundary color.
 
     Return Value
 
     The function returns a nonzero value if the pie is drawn
     successfully; otherwise, it returns 0.
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