NAME
       WritePixel -- Change the pen num of one specific pixel in a
                     specified RastPort.

   SYNOPSIS
       error = WritePixel(  rp, x,  y)
         d0                 a1 D0  D1

	LONG WritePixel( struct RastPort *, SHORT, SHORT );

   FUNCTION
       Changes the pen number of the selected pixel in the specified
       RastPort to that currently specified by PenA, the primary
       drawing pen. Obeys minterms in RastPort.

   INPUTS
       rp - a pointer to the RastPort structure
       (x,y) - point within the RastPort at which the selected
           pixel is located.

   RESULT
       error = 0 if pixel succesfully changed
	      = -1 if (x,y) is outside the RastPort

   BUGS

   SEE ALSO
       ReadPixel() graphics/rastport.h